I think the biggest problem plaguing Smash stages in general is that the devs want to use stages that fit the "spirit" of the series. And just like bootstrapped theme songs like "DK Island Swing" and "Brinstar" and "Green Greens," the first stage of a game usually encompasses this feel pretty well.
The big problem comes when they start porting the old stages up at the same time. We've had our spirit of the game stage, and we've still got it. Now break out and explore the possibilities more!
Of course, this gives Metroid no excuses, seeing as how 5 out of 6 stages have all had a magma/fire theme to them, when all but two are ostensibly different places.
Really, they spend more time experimenting with "gimmicks" for the stages, and don't seem realize that we like the themes, layouts, and atmospheres of the arenas far more than we like barrels launching people away from the action, or Ridley coming in to not be playable, or the like.