MD
Those shows that have been remastered were filmed on 35mm film, and so had the resolution needed to re-scan at HD resolutions.
The Simpsons was animated probably to a non digital format, and it is possible those original cells and backgrounds were not stored, and so can not be re-scanned.
Considering how many other US series that have been shown over here remastered into HD keep their 4:3 ratio intact (Cheers, Star Trek TNG, Murder She Wrote, Kojak amongst others), why couldn't they have done that with The Simpsons?
Those shows that have been remastered were filmed on 35mm film, and so had the resolution needed to re-scan at HD resolutions.
The Simpsons was animated probably to a non digital format, and it is possible those original cells and backgrounds were not stored, and so can not be re-scanned.