There's a rationale for it, but it's pretty complicated.
Basically, we like laughing at people. We especially like laughing at people when they act dumb, stupid or in an utterly clueless fashion. I'm sure there's a psych major can point out that it makes us feel better or superior to see someone acting in a unintelligent fashion, but whatever the reason, we as humans find it funny.
For the first, oh, ninety years of motion pictures (and the first thirty or forty of television) in the US, this often took the form of making fun of anyone who wasn't a white male. Them lady folks were often scatter-brained airheads, causing mischief and mayhem, leaving their husbands or suitors shaking their heads sadly. The black guy would leap up, bug-eyed, and run down the street after "seein' a spook". The immigrant Italian or Jew would totally misunderstand "how things were done" here in America, and the result would be chaos and (presumably) hilarity.
But as time went on, women and non-whites got a little pissed. They were tired of being portrayed solely as punchlines. Blacks were more than Steppin Fetchets, Italians did more than announce "Now-a THAT's a spicy pizza!", and women did more than accidentally set the kitchen on fire trying to make meatloaf when the boss was about to come over. So pressure was put on to give the non-white-males a bit of slack.
It took a while, but it got better. Positive women and non-whites began showing up, first in minor supporting roles, and finally as leads.
Now, it seems the pendulum possibly has swung too far the other way. It seems more and more difficult to portray a non-white or a female in anything BUT a positive light. Part of that has to do with the limitations of the media. We all know that people can be pretty complex creatures. But on TV or in movies, characters get distilled down to a few basics. The bad guy sneers and shakes his fist, the receptionist cracks her gum and says "How ya doin'?", whatever. So if they show someone who's dumb or scatterbrained, that trait is going to make up roughly their entire personality. So making the black guy dumb, or the woman dumb, means that's pretty much their entire role - "the dumb one".
That's where the activist groups come in. Having spent so much time and effort smacking away all the non-positive non-white-males on TV and movies, they still believe that having one will give the implication that ALL women are weak-willed, or ALL Italians are mobsters, or what have you. The fact is - many people still DO hold these stereotypes, and they don't want them reinforced. And since the groups are helpless to go after the people with the stereotypes, they go after that which they feel is reinforcing them - the negative portrayls on TV.
Problem is - we still want to laugh at dumb things on TV. But you can't make them non-white or non-female anymore. Only one choice, then - make the white guy dumb. Everybody's happy.
Has the pendulum swung too far? Not sure. Probably. Eventually it'll go back. And I don't mind taking some of the "white guys are dumb" weight for awhile. Everybody else shouldered it for a lot longer. And I certainly prefer this over people who suddenly feel they need a level playing field after spending decades playing on a sloped one, and them finding that, for once, they're the ones running uphill.
Lex