Because I would like to have supplements that appeal to me? I'd like to be able to pick up something for the planes in D&D that isn't completely locked down to a single setting, same as I can pick up a supplement about dragons that isn't tied to Dragonlance or a supplement about undead that isn't tied to Ravenloft? Or a supplement about planar material that doesn't have to walk lockstep with Planescape? To me, that's the shame of it all. That we get all these really cool supplements about all these different subjects, many of which contradict each other because they are telling different stories using the same base material, but, as soon as we leave the Prime, now we MUST follow Planescape canon, all of it, and not a single line of it can ever be contradicted.
To me, that's the shame of this whole thing. That one setting has been allowed to completely dominate one aspect of the game and it's to the point that nothing can ever deviate from that. Eladrin are bad, not because they don't make sense, or because they're mechanically broken, or even that they're badly written. They're bad because they aren't Angel Elves as specified in some supplement that I could not care less about. Replace Eladrin with every single element that Planescape touched on and that's the legacy we've been left with.
I don't know about you, but, I think that's a terrible shame.
