Is because he's your prototypical west coast quarterback.
What's interesting is that McNabb has had some good success in this system without having the intangibles for a WC offense. A lot of these short throws are like runs - - made to get 4-6 yards and maybe more if the WR is good at YAC. A good WC QB should be able to complete around 60-62% of his passes in order for it to work well. McNabb is too inacurate on his short throws to consistently make that work. How many short throws did McNabb throw over the RB/WRs head/behind him last night? I counted at least 3.
Secondly, a good WC QB should be able to get rid of the ball quickly. While I think McNabb has gotten better with this he still tends to hold onto the ball too long trying to make a play happen when it's not there. Sometimes it works, but other times it results in a turnover or sack. For instance, the TD to Celek last night and his beauty of a -1 yard rush that he should've just thrown away.
Lastly, a good WC QB needs to be able to lead WRs and put a nice touch on the ball. The whole idea is to get the ball out quickly to a WR and let them make a play. Even when McNabb throws catchable balls often our WRs/TE have to stop dead in their tracks to catch the ball going behind them, or pick it up down low.
What used to make McNabb so good was his ability to use his feet. Now he doesn't even use his feet and I really don't think he could that effectively anymore anyway.
With that being said, I think Kolb has shown some flashes that he CAN do this. A few more inconsistent games from McNabb, and it's time to pass the torch.