Any ideas why they portrayed Bud so poorly?

Bud started out as a relatively normal 13 or 14 year old. He seemed relatively normal for a teenager, and was able to do normal things like walk around without banging into things.

That changed in the later seasons. By the time he was a high school senior and a college freshman he couldn't maneuver around the house without crashing into things or tipping his chair over while performing the difficult task of sitting at the kitchen table.

Maybe his judgment improved. Alas, no. In the final season he (with Betty as a passenger) ran a stop and sped to the house, and convinced her to play driver and take the ticket from the inevitable motorcycle cop. A few times he went wild with overspending, both to impress girls, but once jeopardizing his college education and embarrassing the school.

So why would a show degrade a portrayal like this? The show won several Emmys and had good ratings. It ended because Robert Young wanted it to end, and actually continued in prime time reruns for a couple more seasons. I guess it was because they were running out of ideas and needed more scripts. Gray, the actor, may have told them he could do physical comedy and not get hurt, and they said fine, go ahead. It's really hard to know for sure, but we know it's not unique. Beaver sure regressed or at least didn't progress. More toward the present day, Two and a Half Men's Jake was a normal, pleasant little boy who was turned into a dumb middle-schooler, and an absolutely obnoxious teenager and beyond.