Manufacturer licensing also makes it much harder to produce new car kits. Car manufacturers demand licensing fees for anything that bears their logos and/or proprietary designs, and as recently as ten years ago they didn't. These fees must be made up in kit sales- which makes kits more expensive for consumers, causes lower sales, and makes it more difficult for kit makers to recoup their initial investments. It's simply more expensive now to put out a kit (because of the additional licensing fees) than it was ten or twenty years ago- so they're less profitable, and we see less of them. Tamiya (and Hasegawa, et al.) can only do the sensible thing, and shift their focus to more profitable products- like R/C and military kits.
Car manufacturers do not care at all if kits of their cars are made or not- but the days that they would let their designs and logos go for free are very much over. Not that they wish us ill, but they know there is no real money to be made on models- the existence of models does not boost car sales.
Bandai also has the additional advantage that their license holder sees real profit from Gundam kits- as a not insignificant percentage of the total profit realized from the Gundam license. If Gundam kits are unavailable, the Gundam license owners don't make as much money. If Ferrari kits aren't available, Ferrari will easily make up the 'loss' with licensed Tshirts, hats, cellphone covers, and of course sports cars.