If we don't overthink it, it's simple, one guy is more developed than the other at this point. Anderson is still a developing young player that is not going to contribute much offensively, and Pop doesn't want him to. (Or at least Pop didn't want him to originally, lately Kyle has been more aggressive, there were too many games where he was just too passive.) But he's still developing and therefore will be up and down offensively and not contribute much. Precisely because of that, you can't let him get away with the slightest issue defensively.
On the other hand, Simmons is a developed athlete with a game that is growing with experience, and able to make an offensive impact right away. Since he is a factor offensively, you can maybe let him get away with one or two mistakes before you need to do something about it.
Beyond that, I think Kyle was really not meant to play real minutes in this game. Simmons was ready to sub in the 1st Q and then Pop surprised us giving the nod to Kyle in the 2nd bc the match up was with Budinger, a tall bench SF guy. So Kyle got the nod over Simmons just on defensive reasons alone. Then in the first play, the very first play he got in the game, the poor guy (Kyle) gets hit with a backcut by Budinger and a score. If the only reason you are playing is for defense and you don't do your job you will get subbed out. End of story for Kyle.