It's the same reason MUs suck at combat and yet still won't wear armor; mana serves only those who humiliate themselves. What better way to humiliate yourself than to pursue a career where your life is constantly at risk yet the only thing between you and the dragon's maw is a silly robe and a wooden stick? "Hahahaha," says mana as the orcs rush your ridiculously resplendent conjurer, "ok, here's your magic missile." Mana sure has an evil wit.
|Pointy ears and hat?! Double the laughs!|
Silliness applies to gnome illusionists as well: they've got those big-ass noses to make the mana smirk. The forces of magic, however, apparently have an aversion to the hirsute--who doesn't?--thus halflings with their hairy feet and the profusely bearded dwarves are unable to focus the eldritch powers at all.
*as per the PHB 1978, half elves can only achieve 8th level while full elves can achieve 11th and humans are unhindered in their advancement