as for me there are a few reasons
/ one is that i can carry a bigger blade when it fold up into my pocket as where a big 8" plus solid piece of metal in my side.
/ price is another. i can find a fixed blade in a good carbon steel 1095 for under 75$
/ blade shape and size are another. i like the blade shapes on normal spyderco folders, the ark is the right size with a weird shaped blade and handle, the street beat is huge for me, and the enuff is close but too expensive for something i will rarely carry.
/ the way it carrys is another, i can have a 3" spyderco folder in my back pocket and it not bother me to sit on it, a fixed blade is not comfortable to sit down with. another is i can take my folder out of my pocket and not carry it in to work, if it was a fixed blade i would have a leather sheath because they look nicer, i would have to undo my belt and redo it to take off. not convenient at all.
In-Possession - Para 3 S35VN, Para 3 G10 M390 Red, Dice XHP, Lil Native S30V comp B, Manix 2 M390 G10 BR, PM 2 M390 DLC BL, PM 2 K390 RG, MKM Voxnaes Isonzo N690, Benchmade 555HGBlu, Ontario Rat 2 Gr G10
On-Deck - Manix 2 DLC ?, Brouwer, Lil Native compression 20CV/M390
Loved Steels - M390/20CV, S35VN, BD1N, XHP, S30V, N690, K390