I can tell you why they kept some of the original scenes, but the second question i don't understand.
there are proberly 3 main reasons:
1. Its nostalgic and recognizable.
2. Some of these scenes are just really good and allmost priceless, such as the writing on the board and the DOW at the car.
3. It shows some character development, if you don't know the simpsons you would proberly know the roles of the main characters, as you see Marge is the typical mother, lisa the smart kid, bart the little devil, maggie the little cutie and homer the dumb one....
I've heard that recent Simpsons episodes have been following Family Guys style of humor. I like Family Guy, but I think Simpsons definitely need to hold on to their own style of humor. They need some new writers.
Ya they changed it because they are trying to reach a new TV audience. If you look at TV habbits, many things come back in generation gaps. the first generation that watched the simpsons watched terrific political and social satire which at the time cartoons were sorely missing. Then the simpsons became the staple cartoon, and thusly Knockoffs were created and directed towards a new generation, and that was in 1997. 97-2000 were terrible years for the simpsons, they milked it for all it's worth, and will continue to do so. The Idealism of the simpsons has died and replaced with the recognizable and repedative themes. Maybe if our society wasn't high on marijuana maybe we wouldn't be giving Family Guy and the new Simpson's writers the time of day. But don't forget that certain fads trickle down the age ladder, so who knows maybe the Simpsons and Family Guy's main viewing audience is 13... I for one view the simpsons as 91-96 , everything else seems generic.