[Rhod the Red]

I'd say it's either an outright racist reason(anti-Human paranoia) or it's a kind of literalist Creationism reason; IE; Humans are the Third Kind, the Third Race to be created. But created in the Middle-earth continent.

They're not native to Valinor, therefore don't deserve to move to it. And of course until the dawning of the Fourth Age, Man needs to be allowed to learn his own ways, make mistakes, etc. In preparation for the Dominion of Man.

So I'd say those are the two(3?) main reasons why.