For me, it's also because I have a persistent game world that I've always used. So if a tabaxi shows up in a game then they have always existed and will continue to exist. But as the saying goes, once a camel gets his nose under a tent flap the rest of the camel is soon to follow. Allow a tabaxi? Then you have to allow a dragonborn, tieflings, aarakocra, firbolgs, harengons, whatever other species someone wants to play. Fine if it's what you want, I don't. That, and every species will ultimately just be represented as having some aspect of humanity that any human can also focus on. Species seems to fade into the background of pretty much every PC, including my own, after a little bit.
I allow exceptions now and then but it has to make sense and there has to be justification for the addition. I can also imagine a campaign world where Mos Eisley's Cantina wouldn't be out of place. It's just no my campaign world.