Bart didn't start out as he looks today. When Matt Groening first started drawing the characters they looked as ugly as the drawings you probably did when you were 5. They were very rushed and looked fat and dodgy, a bit like today's Homer, but not of high quality.
The reason for this was he didn't have a large art team so to speed the process up the characters had to be rushed, or it would have taken years to make each cartoon.
As the years went on the characters started looking more realistic and smoother, as the show got more viewers.
Originally the show wasn't very long and was just there to fill an advert spot. Later, it got a spot on the Trace Ullman show, but these were just shorts so didn't last long. When it got it's own spot on television, the characters had developed to what they are today, as they had to be high quality to compete with other shows.
The cartoon today keeps with the high quality, but promotional images are even better and use complex shadows to make them look very realistic.