I think it has to do with him not getting minutes and then some unexpected production in the playoffs.
Starting last season, he played 39 games averaging only 8mpg. I don't think anybody was expecting him to get any meaningful minutes in the playoffs. Then Bradley goes down, and he's suddenly getting 20mpg, and he didn't look bad, especially that first game when he chipped in an unexpected 10/4. Now he never was as productive in the rest of those playoffs, but his per-36 production of 9/6/2/1 with 36% from 3 was very encouraging for a rookie who couldn't crack the regular season rotation and wasn't expected to contribute. Man if he can build on that...
Similar thing this year. I thought he was decent as a backup guard. 12/7/4/1 per 36 not too shabby. Improved his shooting %, brought down his turnovers. If you're an optimistic homer (like myself), there's enough there to feed your enthusiasm. If he just got minutes...
Then again, in the playoffs, he had those good moments where he really contributed. 12/6/4/1 and +25 in an OT win against WAS, 16/7 and +15 while looking like the only player with heart in that Game 4 blowout against WAS. 8 rebounds +13 in another playoff win. Other games with big +/-. Good things seemed to happen when he was on the floor. He was a spark plug and his production improved from last year's playoffs.
Plus when players do things you don't expect from their position, in Rozier's case it's rebounding, it can seem like a major asset even if the rest of the game is lacking.
So I can see how people can be excited for him, especially with how excited some of us get over the C's young players.
Now don't get me wrong, I don't expect huge things from Rozier, not expecting for him to develop into an All-Star, but part of me thinks he could be a valuable role player and would probably look a lot better on a bad team where he can consistently get big minutes and shots. Like if he was getting 30+mpg on LAL or PHI he'd look more like Jordan Clarkson than what we currently see.