How can anyone be so dense as to not pick up on the similarities?
When SOTN came out, it was noted for being similar to Super Metroid. Same sense of exploration, open-ended level designing, acquiring new items/objects/abilities/upgrades to get past areas you couldn't get through the first time you see them, and then of course the map system in SOTN and later games is very similar to Super Metroid's.
They're called Metroidvanias for this reason: Castlevania games that resemble Super Metroid and the series' later games.
As others have said, "Castleroid" is an idiotic term. "Metroidvania" at least explains something off-hand; you have "Metroid" and "vania" in the title, and when you think of either of those two words, chances are you're not going to think of anything else besides Metroid and Castlevania, 'cos what else has those two words? "Castle" and "roid" are very non-descriptive, and to echo other statements, it resembles "hemorrhoid."
Plus, from a grammatical standpoint, Metroidvania makes a hell of a lot more sense. Why? Because the first part of the word is being descriptive of the latter: it's a Castlevania game that's akin to a Metroid game, thus, a Metroid-esque Castlevania game.