It feels like a choice, honestly. They've got a race creator tool for the players, and Polyphony almost certainly have a more powerful version of that for themselves so it's not like creating races is super hard or time consuming. They've got the bones of a campaign from GTS if they wanted to use it, plus all the historical races from older GTs that they could just copy if they were feeling lazy.
I struggle to see the impediment to just throwing a bunch more higher class races in, because it's not like the ones that are in there are highly crafted and polished as it is. And so given that it feels like they did it on purpose, the obvious question that raises is "why would they do that?"
There are no happy answers to this question. Even if the truth is that it was rushed and they just ran out of time, running out of time for the basic campaign mode in your game that's been in development for five years is... not great.