The undead can't be put off so easily. Trolls have some race spells (Shadowguard) but an Undead priest can use "Devouring Plague" a useful move in general. Also Undead have "cannibalism" which helps restore mana and hp lowering down time. There are a few other reasons like staying under water for longer and crap but I think those are the two biggest points. Truthfully most people I've spoken with would disagree with troll and say an undead priest would be best but I think mostly for shadow spec. As for Holy spec, I would almost say you're SOL because a lot of people are favouring Paladins over holy spec priests now. A paladin/shadow priest combo seems to be a popular thing right now.
Also shadow priest are good at PVP and solo PVE but they have the ability to provide minor group hp and mana healing through "Vampiric touch and embrace." I'm not saying holy spec is useless but I'm shadow spec and I get picked before holy on even my most bad days.