It's important to have good defense so your players will play clean defense instead of fouling because of inability. Shot blocking is especially important to have at a reasonable level, because if you Center has high ID but very low SB, he will try to contest all shots, but will foul while doing so because he can't properly block a shot.
There's a hidden trait that influences how often a player fouls and is fouled, and that you can change. What you can change is buy players that have proper defense or just don't foul a lot (look at past season, if they average less than a foul per 10 minutes you're pretty safe). Or you train your own players with proper defense.