In the far past magic was considered to be more powerful than non-magic. Unfortunately this lead to too many players wanting to play magic users. Thus, if a fantasy story was to be transfered into a game setting, and if the game designer wanted to allow everybody to potentially play a magic user, he had to balance magic.
Mana is the traditional choice. The magic user can be more powerful than non-magic users, but he exhausts very fast. This was a simple mechanic with a reasonable simulation aspect that introduced interesting decisions in table-top RPGs. Great.
Unfortunately it doesn't really work with modern MMORPGs. The problem is mostly with PvP. (But while raiding the mechanic is problematic, as well). The fact that dying in most PvP mini-games (like battlegrounds) is mostly meaningless, makes the mana mechanic useless.
If the magic user were overpowered while he has mana and underpowered while he has no mana, the result would be a very boring battleground: The magic user would defeat non-magic users until he is out-of-mana, at which point he is defeated by somebody else. Revive. Repeat. Very boring.
Because game designers nowadays cannot (are not allowed to) think out-of-the-box, they didn't replace the mechanic, but iterated it over the last decade. This lead to magic users that have almost infinite amounts of mana and are actually balanced while they have mana. Should they run out-of-mana, they are extremely underpowered, but since this almost never happens, it's not a problem.
This is an unnecessarily complicated mechanic that doesn't introduce interesting decisions and doesn't even balance powerful magic. If you could play a PvP-specced mage in a WoW battleground who had endless amounts of mana, you wouldn't feel a difference, really.
Ok, you could try to arcane-blast everything to death. But actually .. isn't that what's happening and forcing Blizzard to re-balance arcane blast all the time? If a mage were balanced while fighting in a mana-saving way, he were overpowered, because he could just as well fight in a non-mana-saving way. And, of course, most mages would do just that. Which would cause lots and lots of complaining about OP mages in the forums. And this wouldn't even be unreasonable.
The mana mechanic doesn't work in a PvP game that has 30s respawns. It requires harsh death penalties (remember tabletops!) to work. Thus, we could either introduce harsher death penalties, or we could use a different magic system.