I think that:
1, we don't have a rim protector and that is a huge part of team defense, not individual defenders.
2, In the playoffs we're playing against the very best teams with GS and CLE on a whole other level compared to the rest. Cleveland (especially Kyrie) was making shots against very good one-on-one defense. You can't do anything but tip your cap when that happens. If we had doubled him, they are good enough to swing the ball to an open man where an even better shot opportunity might be there. We can't double Kyrie, LeBron, AND Love after all. Finally with LeBron you really you could count the number of guys who can guard him one-on-one with one hand.
3, When our offense struggles and misses shots, it gives the other team more transition chances. Teams with good rebounding get more opportunities from offensive rebounds.
4, In the playoffs, every weakness is gameplanned for and magnified.
5, Isaiah isn't good at defense.
6, Our best defensive lineup has too many holes on offense and our best offensive lineup has big holes on defense.