They're probably not fans of the CTTR characters, though they drew the art in the OP last year. Which is unfortunate because they fit really well with NF's cast (and I'd argue the 3 of them are better characters than 3 of the NST's characters, but a lot of people don't see it that way).

Hopefully Beenox don't take this as most people not wanting the CTTR characters in. Most fans want them in as well.

Or, again, the post is old (2018) and the artist just didn't think that it was possibility CTTR characters or the now-confirmed CNK characters would be in the game because they didn't know the scope of Nitro-Fueled, so they just drew the NST characters which were already very requested at the time. They did draw the CNK bosses last month (including Fake Velo).