Has anyone heard why they have a different duration than the corresponding regular corpses in the first place? Is it a stubborn tech issue that happens to span several games, or do the developers have a reason to want it like that?

it's been like that since F3. WOn't be fixed most likely

well the same issue plagues fallout new vegas except since obsidian abandoned the game there is no chance at a fix but with bethesda im sure they will come up with a fix who knows maybe once they do they will take over fallout new vegas and give them the fix as well.

No, it was the same in Fallout 3, developed by Bethesda.

What I'm asking is why.

Nobody knows?

http://www.gamesas.com/index.php?/topic/1303958-bug-ash-piles-bloat-savegames/page__view__findpost__p__19624550

So certainly not a stubborn tech issue. Either it's such a low priority that they never get around to it, or it's intentional. Can you think of any reasons to want it that way?

When the time comes, those can be modded.
Ash and Goo piles are activators and wont ever vanish unless scripted to in the base item.
Fallout 3 has some good scripts for Pile Deletion, hopefully still on the Nexus.