Mainly, SUV's were not designed with any Circuit in mind. No part of them was ever built to race and many people don't aspire to take their Durango's to a circuit and count lap times.
There is no racing cup for Porsche 911 Turbos.
The Lamborghini Countach wasn't built for racing, neither was the Miura. Neither is the Ferrari 599... nor the Corvette Z06 nor the Subaru Impreza WRX.
The fact that Corvettes and Subarus race signifies nothing. These racers are purpose-built, and are
not equivalent to their road-going counterparts. The differences are because neither car is built to be as fast as possible on a racetrack, but built to be useable on an everyday basis on the road.
Yes there are a good deal of cars that don't have any place in GT5 it would seem.. but they are cars and I'm sure someone somewhere wanted to know how it would drive on the track against other cars.
I was at one trackday where we had a Toyota 4Runner tripoding between cones. Not the safest thing in the world, but if you know what you're doing... fun.
(warning, large photostitch)
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If we're arguing from the performance standpoint, SUVs nowadays are quick. A top-level Cayenne or X5/X6 can lap the Nurb in the same time range as many mid-level sportscars. An X5 with a stock suspension and a V12 racing engine lapped the Nurb in 7:50. That's a ton heavier than a Veyron with 300 hp less, and it's only 10 seconds off.
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No, SUVs aren't as dainty around the track as a sportscar... but again, if you exclude everything that's "not as good", you're left with just ten or twenty road cars and a whole bunch of racers.
I don't know about you, but I find racecars boring. Too much speed. Too much grip. Too much downforce. Road cars are a challenge... understeer, oversteer, poor grip, no aero...
It's always more fun to drive a slow car way too fast than to drive a fast car... errh... fast.