i thought the 2-3 zone was made specifically to discourage inside shots and encourage perimeter shots?
As Chris said, the problem is that you reduce your perimeter d too much with a 2-3 zone. This allows for better passing lanes, which allows your opponent to move the ball around at will and find their best match-up faster.
I will go even further on the issue... You will notice if you watch a look inside/low post game that their are a fair number of short jump shots (maybe even close to 50% on some occasions). I gather that the decreased outside d in a 2-3 zone also makes it difficult to defend these types of shots.
Run of the Mill Canadian Manager