The main reason for the lack of shadow priests is that Smite spam does just enough damage to allow priests to DPS where needed while staying specced for heavy healing.

I'd say that's a bit of a newish reason to be the "main" reason. The main reason is, as previously mentioned, shadow priests burn through mana like it's going out of style, which hampers their DPS significantly. The fact that a 21/30 priest can put on the leftover damage gear they've collected and do decent, if unspectacular, DPS when required is just a bonus of 1.10.