I mean how they are formed and how they organized (which they don't). The Planescape Monster book didn't say if they are born or if they are created or if formed out of dead CG people (Or I didn't remember reading it).
And the eladrins just being CG angel variant who look like smushed elves and sprites. The flavor wasn't strong outside of Planescape. Eladrin for the most part served only 2 roles before 4th: an enemy for evil PCs to spice up their adventures and to sot fill the CG paragon.
Their flavour was a little weak in the MM, but I haven't read
Planes of Chaos or some of the other Planescape lore. They could have been greatly fleshed out in that setting.
There could be tonnes of eladrin lore I just don't know because I'm a Ravenloft and Dragonlance guy.
They were so easy to co-opt. Same with Archons. In Planescape, they were important. Everywhere else..... eh.
Which isn't a great excuse.
"Hey, drow are only really big in the Realms and no other settings. Let's dump them and squish them together with shadar-kai
So the designers probably saw the gamble was worth it. Make them fey, make the PC version the novice version of eladrin, and run with that. Couldn't be worse that a dozen types of elves, right? Or having dwarves be better wizards than PHB elves again, right?
Oh elves. You are so bad, elves.
Honestly, the co-opting of the outsider eladrin wasn't a *big* deal. It's odd but it's arguably an additive change. They're taking this fairly little used race and saying "hey, these dudes also have low level guys the same power level as PCs. And they might be the origin of elves."
Okay, that's cool. Because it doesn't really change anything. Not really. Because all the old stuff still exists and applies and the new stuff is just added to the mix.
And then the game says "oh, and eladrin can now all teleport. That's their thing." Okay... little more odd. I'm sure it wrecks someone's eladrin storyline somewhere or a prepublished adventure, which relies on an eladrin being trapped in a box. It's a revisionary change, but it's not a big revision as that's likely a very minor part of the experience.
"And eladrin live in the feywild now." Well... also not much of a change. Arguably they could live in two places; humans sure seem to live everywhere. Eladrin could have originated in the feywild. Sure.
All that is good. It's a change and addition, but none of that is really problematic. And even if they had said "hey, we want a new fey race in the game that's an elf plus. And it's a weaker variant of eladrins." Okay, that's still workable.
The problem, the only problem, comes when the game came and said that all high/grey elves were and always had been eladrin. That eladrin were the one and only wizard elf. Because that's a whopper of a change.