Because he's not playing to his strength...
When he is on the left or right block facing up and can do one move and power his way to the rim he is completely unstoppable, to big to long to strong your only hope is to get out the way or foul him and you go to foul him hard cause he can power through pretty much any player in the league.
But for whatever reason, the bucks just don't play him like that enough...and they NEVER play him like that in crunch time...I guess because Giannis doesn't want to play like that...since Jason Kidd put it in his head he can be a point guard