Speaking to Eurogamer, Greenwalt explained that the technology used in the Warthog simply doesn't exist in any of Forza's other cars – notably its "crab walking" four wheel steering – meaning Turn 10 would have to build it from scratch. It's not that the Turn 10 team is incapable, Greenwalt noted that they have the specs, but it would be "a big investment" in order to support a single vehicle.
The final straw, apparently, is that there's no way to simulate the Warthog's tires. All of Forza's tire physics are based on real-world tires, meaning that each tire in the game should react to road conditions in a (very) realistic way. Said Greenwalt, "they didn't give us tires from nanotube technology from 2525."