Dude... see my previous post. It was due to restrictions on violence in children's TV.
Keep in mind that the classic T&J shorts were made for movie theaters, but the 70s ones & later were made for saturday morning television. THAT is the key difference. In addition to legal/political restrictions, being made for TV also imposed significant time & budget restrictions. When the classic shorts were made for the theaters, they had months to make each 6~7 minute short, and months to get their investment back (thus allowing for bigger budgets). In the TV era though, they had to come up with about 25 minutes of cartoon EVERY WEEK. On top of the labor aspect of that, they got significantly less finacial return for that 25mins than they did from each 7min short. Less return means smaller budget. Smaller budget + more content + less time = significant drop in quality.
The time/budget restraints were also why jerry got a bowtie. Having a nice clean line under his head allowed for easier use of "limited animation", where they would just animate the head/arm/whatever for a new frame instead of the whole character like they did in the classic shorts. (Ever notice on yogi bear cartoons how his moving arm will often be a slightly different shade of brown than the rest of his body? Having something like a collar to seperate the parts makes such minor inconsistancies much less noticable. This is why nearly every character in all the old hanna barbera shorts (ie, yogi, huckleberry hound, mr jinx, etc, etc) have some kind of collar. Since the head moves almost all the time but the body only moves a fraction of the time, this allows them to easily just animate the heads in each frame & not worry about precisely matching the color of the body.)