He doesn't want them to be likely to be played. Only two centaurs over twenty is fine, because seasonal races are supposed to be very rare. So rare that Thera goes centuries without seeing them.

It's for the same reason that wood-elves have a large experience penalty despite being a not all that great race, stat wise. Because wood-elves are rare, rarer then humans or arials. It's part of how the admin creates the background for everyone's roleplay. I would think you'd support such a thing.