Because the Moral Guardians believed that they were awful, sloppily animated, half-hour long commercials for toys that would brainwash their children into liking. And they may have been somewhat right.
There's a Calvin and Hobbes comic that sums up most 80s Saturday morning cartoons pretty well: Calvin is watching TV, and his dad says, "How can you stand these cartoons? They're just half-hour long animated commercials for toys, and when they're boring, they're preachy. These characters don't even MOVE, they just stand around blinking. What kind of a cartoon is THAT?" After his dad leaves, Calvin says, "Meet my dad, the Gene Siskel of Saturday morning TV".