To my basic knowledge of the situation, this is how I see it:

Killing Floor
1. Has been around for years and has a loyal following, thus increasing word of mouth about it.
2. Has an insane amount of user created content.
3. Great co-op and online multiplayer gameplay (fun leveling system too).

Serious Sam 3 BFE
1. Still new to the market (barely 2 years old I think).
2. Since it's still a new game, user created content is still growing.
3. Focuses on the singleplayer story more than multiplayer.

I could be entirely wrong, and there may be a reason that completely eludes me as to why Killing Floor may be selling more.

My general fear for Killing Floor is from the fact that Tripwire didn't port it themselves. It was Ryan "Icculus" Gordon that ported it, or at least a lot of it.
If this is the case, then how much control or understanding does Tripwire have over their Linux port?
Is it enough to produce bug fixes in the future, or is KF going to be left as-is right now to infinity and beyond?

Share it with me though, if anyone has more insight or information on how things are going with Tripwire and Killing Floor.
I've clocked in about 40 hours of gameplay with Killing Floor in Ubuntu so far, hope to have more.

Some bugs I've encounted in KF:
1. Stalkers aren't invisible.
2. Player models and ZED models become see through without a light source (e.g Scrake/Steampunk skin).
3. Moon Base has some really messed up textures. If you switch on your flashlight, a block texture just covers the parts of the map you shine it on.
4. Welding gun on KF-Manor makes frames drop to almost 1 fps.