In Diablo 2, all of the casters are basically either blasters or summoners. Blasters, as noted, are weak. Summoning is rough in a game where, to put it in D&D terms, you're often facing encounter levels WAY above your ECL, based on the presumption of near unlimited fast healing (which said summoned minions can't generaly benefit from) and the ability to run back to teleport back to town constantly. The whole death is a minor inconvenience thing also makes it less useful to have minions to meat shield for you (and oh god, the lag they can cause...). It arguably takes MORE permanent investment to make a caster than non-caster, since damage depends entirely upon skill points into specific spells and their synergies; warriors can increase damage just by finding a better weapon or increasing stregth. Compare with D&D, where most casters can daily tweak/change their spell list and the spells require absolutely no investment beyond the slot (compared to say...the augmentation system of psionics) and noncasters only get a few feats that become locked upon selection. Also, later on in Diablo 2, energy resistances are extremely common, physical ones still not much so. And the fact the monsters come in droves and have insane health actually makes the whole "I can do this all day!" schtick meaningfull.
What few battlefield control spells there are are weak, very frequently ineffective or difficult to use (am I the only one that ever accidentally bone prisoned myself in with a bunch of enemies instead of coralling them apart from me?

), and extremely short in duration. Save or lose/suck/die is completely non-existant, the closest things to it are Conversion and curses that many things are plain immune to, and last very shortly and have dire consequences (confuse curse is a good way to suck in every enemy from at least 2 screens away very quickly!). Closest thing to a true save or lose, without hte save part, is Static Field. Which is why it's considered such a good skill...
I guess the TL;DR version is: D&D and Diablo are so completely and utterly different in every concievable way, it's not a surprise that balance works differently for each system. Although my analysis is based solely on D2, never played 1.