1)Money, obviously. I'm not going to buy a knife if I can't afford it.
2)Price. Even if I HAVE the money, I won't pay beyond a certain point. Not to dredge up an old discussion, but there's nothing about a Sebenza, as far as ability and function go, for me to justify paying $400 for it. Yes, I know it's well made with super tolerances, but it's not superior enough to justify the extra cost for me personally. The odds of me needing those extra tolerances over the tolerances of say, a $200 Spyderco or ZT(for example) are about the same as getting hit by lightning every time it storms, so I just can't justify paying that much money, especially when (IMO), the Spyderco or ZT will be a better looking knife as well. And when you start getting into Hinderers and even higher priced stuff, well, it applies even moreso. And as someone else said, if I'm at the point where I'm willing to $400+ to drop on a single item, I'm more likely to get a gun than a folding knife.
3)Aesthetics. If I think the knife is ugly, I won't buy it. This is the main reason I don't own more Spydercos, I know they're great knives and well priced, but so many of them look so similar, and it's a look I'm not extremely fond of to begin with, even if it didn't have the Spydie-hole that I dislike.
4)Quality. This one I'm flexible on. I generally prefer my knives to be decent quality with at least decent materials(8cr13MoV or AUS 8 is fine, 440a I try to avoid), especially if they're a knife I might carry/use. But I will make an occasional exception and get a lesser knife, if it's cheap and has something about it I find appealing . For example, I am fond of the aesthetics of the karambit, so I have a S&W brand one, likewise the S&W "MAGIC" assisted opening system is fun to play with so I have one of those, neither are carried by me though.