Zardnaar
Legend
I think what's less important than why the DM isn't allowing the race is how the DM handles it if/when someone wants to play it--and to an extent how the player handles the DM's banning it. There are ways that lead to productive conversations, and there are ways that don't.
Pretty much PHB +3 ask for anything else no flyers is my current rule.
I said no to a Triton, yes to a Dragonkin.