Three reasons, in order of importance.
1: Racing Modifications dissappeared. In fact, most of those race cars that were racing mods of normal cars (the Taisan ADVAN AE86, all those Group A R32 GT-Rs, all the BTCC cars, etc) haven't shown up either. to include them would double the amount of cars in the game.
2: NASCAR Licensing. The original Taurus racing mod was Mark Martin's Roush Ford: this was changed to a generic scheme after EA got up all in arms about it.
3: This is a minor point, but the car drove more like the BTCC Mondeo when fully modified than a NASCAR stock car. It had headlights and taillights, for Pete's sake! I loved using it in the Super Touring Cup race for that reason, but it still wasn't "right" for a stock car.
If you think it'll kill the game to have oval-track races, don't buy it, quit wasting my time.