I think the difference between having seven mana (most likely lands unless you play ) minimum and tapping out with all you've got versus having five lands and tapping only two of them is a huge difference. There are only few decks that can achieve the former reliably while getting a true benefit out of it while pretty much every midrange or control deck should be able to do the latter eventually. I think Caves are probably even playable in aggro.
Then there is of course the color restriction, in my cube at least it's far easier to find a colorless spot for such a card than making room for it in the blue section. On top of that blue is a color that doesn't really need more card draw.