You do know how big Skyrim is right? And Oblivion. And any elder scrolls game.
And all this is based off of one book. One freaking book.
To go through the work and resources to make a quest or reference, while making all the changes to vampires (one part of a huge game). It's viable that they didn't. One book does not equal enough to do that much resource.
They made a mistake, but they didn't do it with the ill-intent or laziness you speak of.
Again...
1) Size is not an excuse. If it's too much work for you to make a game this large without half-assing it (because lord knows the Volkihar aren't the only half-assed part of Skyrim), then don't make a game this large. Simple enough. Attempting to make one this large and failing is poor management.
2) ALL THE RESOURCES to make a lore-friendly, game-functioning Volkihar vampire are
already in the game. They took the time to make several resources that would work for the Volkihar, but failed to utilize them. It's either ignorance or laziness.
3) You keep saying "they didn't do it with ill-intent" but you provide no evidence. Don't get me wrong, I'm not saying they went "mwahahahaha let's destroy our own lore, cuz we're evil!!" But did they forget their own lore? Were they too lazy to care for their own lore? I've provided evidence that suggests yes, yes they were. You haven't provided any evidence that says otherwise.
Because there isn't any. Bethesda hasn't addressed the issue at all, as if they don't even realize there is an issue or simply don't care about the Volkihar lore. It's ignorance (unlikely as they remembered the name Volkihar when naming the castle), laziness (possible, I mean the vampires we got were copy-pasted Oblivion ones with Frost resistance) or apathy/they didn't care. (very likely, I mean they didn't even write up a "why" excuse for why the Volkihar aren't as previously described)