Automatics just aren't very popular or common in Spain, so the scarcity of automatic rentals may have something to do with it. But I think it's the same in the UK. I think rental companies figure anyone who wants an automatic, it's probably because they can't drive a manual, so they've got 'em by the short 'n' curlies!
Conversely, have you ever tried asking for a manual from a rental co. in the USA? They'll look at you like you're some kinda freak!
(BTW, I drive an auto - out of choice, I hasten to add!)