I was looking at PokeDiamond and it looks pretty finished. Why isn't it used?
You're sorely mistaken. It's not complete at all. It's far from being complete, in fact.
There's only a few sprites dumped, the battle engine's code is fully missing. Hell, you can't even edit the overworld scripts.
It's not finished at all and it's not even being actively developed because its most active contributor was kicked out some months ago for reasons.
You can see quite clearly that the last time actual progress has been made on it was December 2021.
In its current state Pokediamond is about 21% complete in terms of code that was decompiled, judging by the bot that reports this information inside Pret's Discord server.
Pokeheartgold is even less complete, it's at around 6.4% complete.
Neither of them are usable for full NDS ROM Hacking projects.
That being said, you do need to keep in mind that even if they were in a usable state currently, there could still be some people that would still use the GBA Decomps out of mere personal preference.