The Forgotten Realms wiki page on succubi tell us (specifically in footnote 1) that in 1e, 2e and 3.Xe1, succubi were chaotic evil demons, but then were retconned to be lawful evil2 devils in 4e and have now just been made into generic neutral evil "fiends" in 5e, presumably in an attempt to avoid contradicting any previous editions’ lore.
Does 5e give any sort of in-universe lore explanation as to why they are now neither devil nor demon? The 5e Monster Manual entry doesn’t really explain that besides briefly mentioning that they "can be found in service to devils, demons, night hags, rakshasas and yugoloths", again presumably to avoid contradicting any previous editions’ lore, but without explaining why this is now the case.
This is the second part, which was split out from another question; see: What is the in-universe explanation for why succubi, who were demons, became devils?
1 Actually, the footnote on the Forgotten Realms wiki page only says 3e, but I know it was still true in 3.5e because of Neverwinter Nights 2, which was a video game based on 3.5e. In this game they were considered demons, which is incidentally my introduction to D&D and why I consider succubi being demons to be what they "should" be.
2 I say "lawful evil", because that’s what a devil’s alignment is, but I’m aware that 4e changed the alignment system, so it might not be so accurate to claim they were "lawful evil" in 4e, but at the very least, in the context of D&D overall, they would have been considered lawful evil all the time they were considered to be devils.