why didnt he make moves to get better at the deadline for trading when other teams were getting better?
Because he had every reason to believe the team would play better the second half. And after game one of the conference semifinals, he looked 100% right.
It didn't work out, just like it didn't for every other team but the Raptors this year.
This might be nitpicking, but people often talk about the period after the all-star break as "the second half of the season," but really the season is already about two-thirds over by the time the ASB rolls around. So Danny had witnessed a lot of disappointing play for two-thirds of the season, which is a pretty big sample size, and I really don't think "he had every reason to believe the team would play better" in the remaining third of the regular season. The team dysfunction was obvious to everyone on the outside; it had to have looked even worse for those on the inside, such as Danny. And yet he didn't lift a finger to help the team.
And as someone further up the thread wrote, it wasn't a question of Danny doing his due diligence in the months leading up to the Davis trade, it was a question of Danny doing his due diligence a year or two ago—meaning that Danny should've been finding out a long time ago whether Davis would be interested in Boston. And if Danny would've known, say, a year or two ago, that Davis wanted no part of Boston, Danny could've shifted his focus completely away from Davis and begun focusing on other superstar targets.