L4D levels were not really much more randomized than Darktide

The difference is L4D does not need the randomization in the same way Darktide needs it. L4D has a story told through the levels, through clear arcs via the campaigns they were part of. Each campaign had multiple levels, the first connected to the previous and built upon what had happened, and the last a big grand finale and an extraction/escape. By the end of each L4D game, there is a clear 'ending' to the survivors stories. In both instances, there is the 'final' escape into the arms of the military to a safe zone.

Darktide does not have any of this. You instead play each level as if it was a highly compressed campaign. You show up, have 1-2 big moments, then 'escape', but that means you get all the downsides and none of the upsides. You don't get a proper 'build up' to the finale, you don't get any real environmental story telling or sense of progression--even self-contained to the campaign. The extractios themselves either feel abrupt (with maybe the one exception to the rail yard mission), or you don't even 'get' an extraction. The mission where you kill the Chaos dude with the forcefield just kind of ends after you steal his wallet. The other levels involve running to an elevator, or walking into a transport.

Every l4D campaign was very unique and stood out, and a player could easily tell you what would happen across the campaign and wouldn't possibly confuse the sequences. L4D2 in particular really shines at this. You go from fighting in a 'Dawn of the Dead' styled mall (with a finale where you fuel up a stock car and escape) to a amusement park (with a finale as a big rock concert), to the swamps (escaping by huge ass boat), to fighting through a hurricane (back to the boat!), and then finally making it to the military after running a guantlet off a massive bridge.

This isn't to say I find the game bad. There are some particular scenes of the Darktide levels that look beautiful, Fatshark did great with the mind-bending sense of scale of the cities and foundaries. They did really well nailing the sense of chemistry between the characters. But you are ultimately limited in relying on characters unless you record hundreds of voice lines of potential interactions with the environment to use that same style of story telling.

Just as an aside, one aspect that is being ignored here as well is the modability of L4D. Whether it was skins, guns, entire maps and campaigns, etc. That is a big aspect that also added to longevity I doubt is going to happen here.