The question might be better answered by asking, "why do you not buy." Now that I have all the knives I need, a knife has to be damn near perfect in my eyes on a number of counts.
For example, I briefly thought about buying an S90V Spyderco Military just because the use of S90V is so rare, and doubtless has great edge-holding.
However, Spyderco could have offered it w/ Wave & DLC coat and I would have not been able to pass. The lack of those things gives me enough excuse not to buy.
I thought about buying one of the zdp William Henry 'Gen-Tacs' for carrying while jogging. But since they no longer offer the liner locks that I prefer, only their newer button locks, I passed. If I could buy a liner-lock Gen tac, I would in a heart-beat, but I will stick (pun intended) with my old WM Henry carbon fiber model, with liner lock, pocket clip, and DLC coated 154CM blade till something damn near perfect comes along to replace it.
Also, as I get older, I start to see the advantage of carrying the same stuff, in the same pocket, in the same circumstances. That way I don't have to frisk myself to figure out "now which pocket did I decide to put that newfangled knife in today? Did I use the clip or not. Is it a liner lock, button lock, or shlock-lock?""