The Athletic has a comprehensive article on why the Bucks fired Griffin. I can't post the entire article here as it's behind a paywall (I have a sub but it's against CS policy) but I can post snippets of it.
Apparently this is a decision that had been coming for some time. Despite their 30-13 record, the Bucks weren't happy with how they were playing, and their ability to challenge the true contenders in the NBA.
Why Bucks fired Adrian Griffin and why Doc Rivers could be their next head coach
Shams Charania, Sam Amick and Eric Nehm
Jan 23, 2024
The Milwaukee Bucks’ choice to fire first-year head coach Adrian Griffin on Tuesday was hardly an overnight decision.
It was borne out of months of underwhelming play with internal concerns growing about the severe decline of their once-elite defense, the flawed use of newcomer Damian Lillard alongside franchise centerpiece Giannis Antetokounmpo and a widespread fear that this group, which was widely expected to contend for a title, was likely to fall short if Milwaukee stood pat.
As far back as the In-Season Tournament in December, when the Bucks lost to the Indiana Pacers in Las Vegas and the internal skepticism grew regarding Griffin’s ability to lead, there were strong signs that a change was coming.
Doc is the current first choice, with Kenny Atkinson the fallback.
League sources say Doc Rivers, who is an ESPN analyst after getting fired by the Philadelphia 76ers last May, began to serve as an informal consultant to Griffin at the behest of the Bucks. One month later, multiple sources briefed on the matter now indicate that Rivers is the serious leader for the now-vacant position and the preferred choice of key stakeholders.
The Bucks have started discussions with Rivers to become the new head coach of the franchise, league sources said. Warriors assistant coach Kenny Atkinson, who was a runner-up to Griffin in the Bucks’ head coaching search last year, is a candidate for the job as well if the team is unable to secure a deal with Rivers.
Losing Stotts early hurt him.
Stotts’ decision came about for myriad reasons, but all of the underlying issues seemed to come to a head following a brief verbal argument at a shootaround in Oklahoma City on Oct. 17, 2023, first reported by The Athletic.
League sources said Stotts, who was Lillard’s coach in Portland for his first nine seasons there, and Griffin never saw eye to eye on his role and responsibilities as an assistant and the disrespect Stotts felt in that situation served as the final straw. Regardless of why Stotts headed for the exits, the notion of Griffin’s coaching staff losing its most experienced voice was an indisputable setback.
He started to lose the confidence of the players around January.
Team sources said players began to question Griffin’s schemes on both sides of the floor and the strategy that was being laid out for them each night.
While players were willing to be patient with Griffin as he learned on the job because of the team’s vast personnel changes at the start of the season, their questions grew more significant as the team failed to show substantial growth at the season’s midway point.
According to team sources, the issues plaguing Griffin’s early tenure ranged from putting together strong schemes on both ends of the floor for the Bucks to fulfill their championship potential to successfully communicating his vision to his players for them to execute it on the floor. But the bottom line, given the high stakes of this Bucks era, the concern over Griffin’s ability became a question too big for executive leadership to withstand any longer.
Giannis was particularly upset when they gave up 122 points in a 122-116 loss to Houston, implying that Griffin didn't know what he was doing.
Following that loss, Antetokounmpo spent seven and a half minutes bemoaning the Bucks’ defensive issues.
“Now, defensively, we have to have a plan,” he said. “What is our strategy? Are we going to give a lot of open 3s? Are we going to let them get in the paint? When they go in the post, are we going to stay with ours and play one-on-one? What is our strategy?
“Right now, we are giving everything. We are giving everything. We are giving the 3s. We are giving straight-line drives. We are letting guys play in the post and get comfortable. We’re giving offensive rebounds.”
The team wasn't impressed with Griffin's defensive schemes and inability to leverage the strengths of Giannis and Brook, two All-Defense team regulars.
To open the season, Griffin employed an aggressive defensive game plan that looked an awful lot like the units he ran as the top assistant and defensive coordinator under Nick Nurse in Toronto. After a 130-111 loss in Toronto in the fourth game of the season though, a group of veteran players went to Griffin and told him that such an aggressive scheme was not going to fit with their personnel and suggested letting Brook Lopez, the 2023 NBA Defensive Player of the Year, go back to playing drop coverage in pick-and-rolls and remaining closer to the rim throughout defensive possessions rather than trapping or switching near the 3-point line.
Changing Lopez’s responsibilities helped the Bucks stabilize themselves defensively after a rough start to the season, but they are still struggling to consistently put together stops. To this point in the season, Griffin and his staff had not found a way to scheme around their personnel issues and properly leverage the defensive strengths of Lopez and Antetokounmpo, the five-time NBA All-Defensive Team honoree and the 2020 Defensive Player of the Year.
But it wasn't just on the defensive end that the players lost faith in him, it was also with his inability to integrate Dame into the offense, even though they are 2nd in the league offensively.
League sources also said Lillard has spent much of this season struggling with the way the Bucks function on the offensive end. While he has remained patient with coaches and teammates, there was an inevitable pressure on Griffin from the organization to make the most of Lillard’s talent that validates the choice to part ways with Jrue Holiday, Grayson Allen and three first-round picks to land him.
Despite Lillard being one of just eight players averaging at least 25 points, six assists and four rebounds this season, he has seen his efficiency plummet. From his overall field-goal percentage (46.3 to 42.7) to this three-point proficiency (37.1 to 35.1), he’s not the marksman that he was before.
The 40 point loss to the Cavs appears to have been the final nail in Griffin's coffin.
The embarrassing loss to the Cavs appears to have been a tipping point.
With the newest member of ownership, Jimmy Haslam, in the building, it served as a vivid reminder of the questions Griffin has struggled to answer. The Bucks, who had spent so many nights struggling to find the synergy between their dynamic duo of Lillard and Antetokounmpo, looked lost as they tried to figure out how to build an offensive attack without their franchise centerpiece.
It could have been the perfect time to let Lillard loose, for him to enjoy the kind of offensive freedom he was used to for all those years in Portland and that has been so much tougher to come by in his Bucks experience. Instead, after the Cavs jumped out to a 22-2 lead in the first quarter, Lillard finished with just 17 points on 7-of-20 shooting to go with five assists and a minus-26.
Full article at
https://theathletic.com/5222650/2024/01/23/bucks-adrian-griffin-firing-why-doc-rivers/The TLDR version: He lost the team and the locker room and that was the end of him.