The thing to consider here is what the expectation for "enhanced versions" actually is.
Here's what Platinum added to Diamond and Pearl:
> cleaned up interfaces
> sped up battles
> animated Gym Leader sprites
> graphically overhauled the region to give every city, route and dungeon its own unique character
> (this includes totally remapping a few particular areas that needed it, as well as adding a few new areas)
> made the story more coherent, made the lore and the villains' goals clearer, expanded the roles of characters who were already there to improve their development
> added two more minor characters
> expanded the Sinnoh Pokédex to include all of the missing Gen IV evolutions and improve type variety
> added a ton of postgame content (most notably: battle facilities, the Battleground/Gym Leader rematches, more Legendaries, the Villa)
Platinum was arguably the *most dramatically improved* third version (and Emerald, which is close behind, did many of the same things).
So then let's imagine a hypothetical Grey.
What, of this, is even left to do?
BW already had clean interfaces, the fastest battles in the series, a region where every city, route and dungeon has its own unique environment (and all cities and most dungeons their own unique tiles), a coherent story with clear lore, clear motivations for characters, the most-developed rivals, the most-developed side characters, and arguably more postgame than most of the *first* paired versions of a Generation on top of all of that.
So what was left? Animated Gym Leader sprites? More minor characters forced into the existing plot, which already has far more characters than Platinum? A bigger Pokédex? A bigger postgame?
This is why we got sequels instead. B2W2 DID have animated Gym Leader sprites, more characters, a bigger Pokédex and a bigger postgame - with that alone, they would have been Platinum-tier in terms of content. But that's too little to change... so they made a *whole new game* instead!
Sequels win.