To keep the shows consistent, I assume. (Or a li'l separate from the games possibly.) Same reason they kept Mario & Luigi's SMB1 color schemes, and didn't make Peach blonde yet when the SNES came out.
I always liked how the Mario cartoons used 8-bit sound effects, and music from the games, not many game cartoons did that.
I understand that, but as a SatA.M. fan I actually got the opposite experience, as game Sonic fans used to bug me for liking the Sonic cartoons/comics, and they'd hate on the old cartoons/comics for not being 100% like the games. Oddly, another fraction of the fanbase hates the comics now for being "too much like the games." I even took heat from SatA.M. fanbrats for liking the games & comics. So I've seen annoying fanboy-ism from all sides frankly.
Peepers was quite funny, and adorable. She's voiced by Sandy Duncan whom was from G1 MLP.
I doubt it, the AVGN did try the Shadow game, and was surprisingly more fair to it. In fact, he was more critical of the old GameGear games like "Sonic Blast" & "Sonic Labyrinth." (Though, I actually liked "Labyrinth" personally, as it was interesting relyng on the spindash.) So even he understands not very old Sonic game is great, or every modern one is "bad."