The problem is too many people are anti-social in an MMO lol.
People don't like joining voice because it makes them uncomfortable hearing new people talk, and it's easier to just put these groups on blast in chat if a wipe occurs, leaving without any kind of accountability. Not that there's much accountability either way, but you get the picture.
If you want good groups, ask that people get into voice and look for people with similar experience. You don't have to ask that they've done the 15 (for example) already to join the 15 that you're making, but if you see they've actually completed numerous keys around the 13-15 level (especially the key you're trying to do), it's likely they at least know what's going on in the dungeon.
A large portion of the WoW player base is socially retarded, but the voice thing I mentioned can be pretty legit. A lot of people you meet in this game can be absolute raging retards in chat, but when you suddenly start attaching voices to people it's far less likely to happen. If you get a voice automated phone call IRL you have no problems hanging up, but when you hear an actual human being on the phone you're usually going to be far more polite, even if it catches you at a bad time or it's not something you're particularly interested in.
So yeah. Make your own groups and ask people come and listen. That way you're all on the same page and it's less likely to be a rage fiesta. You're still going to get a lot of people who will never mic up on comms, but I can assure you the amount of people you meet who flat out leave or rage at individuals in your group is going to diminish greatly. You might even make...friends so that you don't have to go through the awkward situation of having to meet randoms to do dungeons with...