I don't understand, he says he's been stuck in Crowhaven for several decades (however old Agronak is), then I noticed there is a burial hall in Crowhaven inhabited by several vampires too. Why haven't his kindred bothered to find a way to free him?

They probably either didn't know he was there, or didn't care. I haven't done this quest in years, but it's likely they, too, are just as deranged as Lord Lovidicus.

That's as good an explanation as any.

I think the real reason is just sloppy writing by Bethesda. It hink they figured: there's a vampire for this quest, so to make the quest more difficult let's add other vampires.

Nothing deeper than that.

I say this as a major Oblivion and Bethesda fan. But sometimes the writing and plot construction is just shallow and sloppy. But that could be said of a lot of major works of art including popular movies.