Think about him in a tactical way. How fast would he have to be to be unique as a heavyweight while not taking a piss at the fact that he's heavy to begin with? Being heavy, he'd only be able to move fast by throwing his entire momentum around, thus making him very clumsy as well. (Which is a definite possibility.) That means a poor acceleration from standing and a Luigi-esque lack of traction. And even so, I don't think he'd be able to move much faster than DK at the most.
Additionally, if he did move like that, it would set up a possibility for a moveset based on being an awkward, clumsy character, but one with great power if you're skilled/lucky enough to hit with his attacks correctly. The Jet-hammer attack seems to support this, as well as the random nature of his over-b and the lag after his super jump. Anyway, DDD has always been kind of clumsy in Kirby's games, but then again, so have a lot of Kirby characters. Besides MK.