Why do people compare Killing floor to Left 4 Dead

I like Left 4 Dead 2 far more than Killing Floor.

Not getting it... You're comparing them yourself...

They are both coop zombie games so that's what makes people compare them.
I played both and I can say that they are very different gameplay wise.

They're both team based zombie shooters. To people that don't play a lot of games it's easy to compare them. It's like if you just started playing platformers you're probably going to compare them to Mario. It's also easy as a starting point when trying to describe a game to other people not familiar with it. L4D is much more well known than KF so you can mention the similarities and then move to the differences.

As for why people like it well they just enjoy it. It's that simple. Some people enjoy bananas. Wave based survival games are pretty common. I don't recall the names but I think Gears of War has a Horde mode and there's the obvious Call of Duty zombie modes.

P.S. I know they're Zeds which are mutants and not zombies. Like I said it's easier to compare first then differentiate ;)

they are both different for the gameplay,the background,the weapons,the Creatures(infected,special infected for left 4 dead,and many variant of zombies monsters and they are more Metalic than left4dead),the gore system ,the Map........ect you cant compare those games

Because :

1. Co-op
2. Zombies
3. Shooting

honestly cant think of any other reason apart from those 3 when comes to comparing them since they are quite different gameplay wise. Still, I had more fun in L4D than KF.

I dunno, they are nothing the same. They both have a group of people shooting zombies and have a fat one that pukes on you. I think that is where the similarities end.

Don't compare those 2 games, there's no comparison.. I have played l4d2 and even enjoyed it, yeah it was nice game, but when I started to play kf... I couldn't stop in 300 hours of game play (and doing all achievements). You need some real skill to play kf, while l4d2 looks too simple when you try to compare it. But you will never enjoy it in last 20 hours, I had a friend who was insisting on playing this game and when I started to get basics,... I fell in love with kf <3

I can't describe the beauty of kf in words... this is not the game for some casuals. While l4d2 can be played by everyone...

I've played only Left 4 Dead games - so as an outside (but well-informed :) ) gamer's opinion:
They are really similar in terms of needed teamplay. I think this is what makes them different from all of the co-op (playing together) shooters. If you don't play along with others in these games, you'll die. And maybe your team as well.
Let's ignore the obvious, but otherwise knowledge-needy info: enemy types, game mechanics, characters, etc. They are there, but as Yzil excellently mentioned: the biggest similarity / different thing is the most important, smaller, "what's it's name" type of things are ignorable.
I'm not sure about this, though: I know L4D games are strictly linear - but you're always on the move while not fighting bigger groups / the horde. Isn't The Killing Floor is kind of Arena based? Defending your position for fix number / type of waves?