There probably aren't any reason that nobody's doing DND Vlogs more than simply because "everybody's doing CoC Vlogs."
CoC is the thing that kicks off the JTRPG Vlog, and is also a BIG part of making CoC so big in Asia compare to other places. That can in theory leads to: 1) More people playing/wanting to play CoC, and 2) More Vlog about CoC. (It's the same idea like Coke, Nike and MacDonald's, and also why DnD is so big in the beginning.) Also, supporting evidence: Goggling D&D live plays have 835k hits, while Call of Cthulhu Live Plays have 200k. That hints that local popularity is different by location instead of brand qualities.
That does not mean D&D videos are by definition less interesting.
(Spoony's Counter Monkey videos (eng)
are interesting, and he does many games including D&D.)
Plus, "all good Vlogs are fake by their essense" due to editing and post-production, of which the JTRPG Vlogs are on average better produced than anywhere else. (Try to watch a bunch of guys playing in their basement as is... Ya'know what I mean.
Less ppl playing & less people making the effort + a better subject (CoC) for Vlogs = Overall less DnD vlogs. Whereas any good video producers should be able to, in theory, make any good vlogs regardless of source materials. (SS Germany propaganda vide... Ooooookay lets not go there.
Trump's campaign ad... Let's just stop there.)
In a sense, if you're into Vlogs, maybe you can think of it as a challenge to make a good DnD one? We can always do with more of these.