because they were horribly implemented and early on extremely buggy. they also seemed to get WORSE later on.....ulduar/wg not so bad....jousting? absolutly horrid, then of course they added a quest with bird jousting in cata ugh
I quite like siege vehicles, they're a welcome addition to the game. On the subject of being too slow, if the ones in Wintergrasp, IoC or SotA were faster then it'd be a bit easier to win, eh? Those 2 bg's I mentioned are a fun difference to constant PVP'ing in CTF bg's, or endless zerging of graveyards.
Also, for those vehicle-type quests through leveling and end-game content (like in the Eye of Eternity), it makes the content different from either gather X of Y/Kill X/Just attack boss for the whole fight.
I thought they were alright in WG, when you actually had to coordinate which side your train of siege weapons would steamroll. Seeing 16 sieges, catapults and tanks rushing at your castle was cool. I liked that.
SotA and IoC fall flat because of the smaller scale IMO, among... other things.
Personally I'm extremely disappointed that they've been relegated to just a handful of quests. They've even had their role in Strand of the Ancients reduced and partly replaced by bombs... They had the potential to really enhance Battlegrounds, but the disdain of a relatively small amount of players has sealed their fate.
There are many different reasons why people don't like Siege Weapons:
If the person driving doesn't know what to do or just refuses it can completely screw the whole team.
Many players are more interested in "pwning noobz" than playing objectively and thus claim that Battlegrounds should just be a giant arena.
Some people take Player vs Player to literally mean "character vs character" so hate anything other than personal combat.
Having to learn additional abilities on the fly can be awkward. And if someone doesn't understand it can screw the raid/dungeon.
Some players just don't like doing something different or not directly playing their characters.
I think one of the problems is that Siege-type Battlegrounds depend on just a handful of players to do everything right. Perhaps a better way to implement them would have been to offer another way of destroying walls/buildings and having sieges as optional, yet more efficient.
I'd have loved to have seen more large-scale siege battles, but sadly I doubt Blizzard will bother with them again.