I sometimes see people wondering why we see so much in the way of SNES ports on GBA, but fewer Genesis ports; usually it's Sonic games being lusted after. Anyway, I have my theories as to why this doesn't happen more, and here they are.
One is color. Though the actual amount on screen varies greatly from game to game, at maximum the Genesis could have 64 colors on screen at once. For SNES that number is 256 colors. The GBA can have up to 512 colors at once. So compared to an optimum GBA game, SNES games are able to at least live up to one half the potential, whereas a Genesis game would be one eighth.
Another, and the most important I think, is resolution. SNES games were almost always 256x224, Genesis games were 320x224, and GBA games are 240x160. Resolution was one of the advantages Genesis had over SNES back in the day, but now that is made moot, and even a disadvantage. Going from SNES to GBA, they lose 16 pixels in width, and 64 in height; overall they're able to have 67% as much screen on GBA as on SNES. However, going from Genesis to GBA one would lose 80 pixels in width, and 64 in height; overall about 54% of the screen would be able to be preserved. Starting from the SNES almost one third of the screen is lost, while for Genesis almost one half is lost. In defense of the Genny, though, there were quite a few NES games put on Game Boy Color, where the resolution was only 40% of the original screen.
So basically, when you compare all three, the strength of the Genesis is its high resolution, but it's the least colorful. SNES has less resolution, but is more colorful. GBA has the least resolution of them all, but it's also the most colorful. If a Genesis game is made to fit on the GBA screen, it's the least attractive set of attributes: least color and least resolution.
Less clear cut to show examples of, but still true, is that SNES tended to do more extra visual effects, too: scaling, rotation, mode 7, transparency, etc. GBA does even more of these things, but it's another way that SNES games are able to compare more favorably with other GBA games than Genesis games would.
I've only looked at it in terms of graphics so far, but I'm sure you can also see that the loss of screen can hurt a game a lot as well, since you simply don't see as much of what was there originally. Genesis games just lose a lot more in the transition than SNES.
As one final visual comparison among their resolutions, the image below is Genesis resolution. The gray rectangle within it is SNES resolution, and the purple rectangle with that is GBA resolution.
December 20, 2002