Because anyone who doesn't love Wizards hates them and thinks they're overpowered. So while the Wizard might get an extra spell slot and have access to 2-5 autodamage per round, he has to beg his buddies to form a wall in front of him so a goblin's sneeze doesn't put him out of the fight. At the same time the Cleric bashes things with her weapon and heals when needed. Why not stick it to the Wizard one more time by letting her rain damage through her god whenever the mood fits.
I've spoken often of the War on Magic in D&D. I firmly believe that it is the goal of all those who prefer strictly martial characters to cripple spellcasting classes, but especially the Wizard.
(This probably sounds needlessly antagonistic, but it's not meant to start fights. It's simply based on observations I've made over time.)