Because it's awkward juggling the two if you want to keep a NFE Pokemon. And I can't think of any reason you'd be using one and not want the effects of the other too.
Only downside is Everstone is usually available early on, and early Eviolite would be OP as fuck. But just letting you upgrade your Everstone near the end-game (around the point that everything is level 30-40) would work just fine. >>38635507
That would be nice too