the simple answer is........ money. they aren't building cars for the average family, who car about fuel economy, etc. they are building cars for rich pricks who will service the cars when they should, and don't really drive them every day of the week (although some would).
but now the gt3 has ceased production (probably because they weren't selling enough turbos). but i am glad that they replaced it with the gt2, a car that, even without the extra power over the turbo, would out perform the turbo due to being lighter than the turbo due to not being 4wd. i'm pretty sure that around a race track the gt3 is quicker than the 911 turbo