I don't know WHY anyone wouldn't. lack of comedy is not a good reason, since they could no doubt make it just as funny as ever REGARDLESS of graphics. I think that if they make a new Mario RPG at all, it's a priority to buy. those games are ALWAYS awesome, and they are released so rarely that you really should look forward to one.
"Yeah! For what it's worth, I'm willing to unleash the power of the pants!" -Quote by Neemon, in Digimon Frontier episode 43
Personally I think cartoony graphics suit the Mario RPG's more than smooth shaded 3D graphics would. If a game with such visuals existed, I'd certainly play it, but I just don't picture it making the gameplay experience any better.