One thing to keep in mind is that the game's already paid for and is being made with a significantly smaller budget than most games these days, it doesn't need to sell gangbusters to turn a profit.
I'm not certain of the timeline but I'm fairly sure this game was announced for the Wii U before its sales woes started, the decision to include XB1/PS4 versions is more recent. Given the similarities between PS4/XB1 and PC architecture, porting shouldn't be an expensive endeavour and might help to make up for the small Wii U user base.
I'm certain the developers are counting on a lack of competition on the Wii U and established audiences on the XB1/PS4. Again, it doesn't need to appeal to every Forza or GT fan, given the low development cost, just a percentage of them.







