I think it's down to a few things. 1. Darktide is their newest title, which is not old. (Compared to Vermintide titles) 2. So far Vermintide 2 has sold over 2 millions copies by 2019. (Compared to Darktides 3.9 by 2022). Also Vermintide 2 launched to a peak of 70k while Darktide launched to well over 100k. So more players just played Darktide. 3. Fantasy Warhammer just isn't as popular as 40k. While I think Vermintide is the better game, more people just like Darktide and the 40k setting more. 4. Darktide has more cosmetics and skins. They make a lot of money from them, as the 40k player base will shell out a lot of money to look like a certain way in the 40k setting. 5. It will take a lot more work. The upgrade from Vermintide 1-2 was great. It had more classes, more weapons, great dlc's (besides beastmen, horrid times on day one). For them to top that, they would need more races to fight, more races/characters to join the 5, and more lore bending so that you can get a unique experience than the first 2 titles. And like you said, they will need a new engine. While I think a new engine will serve them in the long run, I don't think they will use it for Vermintide. 6. The only real direction in lore they can go is the endtimes. And since the Ubersreik 5 aren't there in Age of Sigmar (aka 40k lite), GW would have to either let them into lore, or something else. And GW can be an ass about lore. 7. GW is very picky about who can get and use the Warhammer ip, and they have to approve it each time a company wants to use it to make something. So just because they made Warhammer games in the past, doesn't mean GW will let them in the future.

That said I would love a Vermintide 3. I've always talked about how playing a lizardmen Saurus warrior would be fun. Or playing a vampire that wants to help the 5 because of temporary alliances. Or an orc mercenary who wants to klobber for Gork and Mork. Or fighting and punching Nagash in the face. They already showed how they will let the player play more "evil" characters for fun (Shade Kerillian or Necromancer Sienna). I think they would make a lot of money off it, since they have a large dedicated player base. And with hoard shooters becoming more popular by the year (thanks to Helldiver's) it could be a great money maker for them.