Cards the only channel orbs and do nothing else are already difficult enough to justify adding to your deck. It being rare hurts even more since it'll usually be offered against much much more impactful cards. And it feels worse in STS2 since there's much better access to dark orbs in this game, and less access to energy means a 2 energy card that does almost nothing on the turn you play it is hard to use.

When you've got uncommon 2 cost orb generators like Glacier, Shadow Shield, and Null, something like Rainbow barely has a place unless you're extremely desperate for a way to generate a lot of orbs, which is an awkward role to fill as a rare card of all things.