Why do mages in world of warcraft die so often

Mages are meant to be "glass cannons". Meaning, they do a heck of a lot of damage, but are easily taken out. They really shine in dungeons and raids where they can sit back and do their magic while the tank keeps the mobs busy. In PVP, frost spec'd mages do alittle less damage, but survive a lot better with the slows and Ice Barrier.