I think it's more because they don't want to lock themselves into promises they can't keep. Firstly, there is a lot of dialogue to write and record for the races and classes; don't underestimate how much work that is. Then. Regular 'humanoid' works fairly well, because they use the same facial/bone structure, therefore it makes it less hard to differentiate between animations. Making that work for someone as foxy as a tabaxi... That takes quite a bit of effort. Since they're still working on including the PHB and making all that work, don't get your hopes up for the first release of the full game. Perhaps in a DLC.