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 S30V, Para 3 S35VN, Para 3 Red G10 M390, Dice XHP, Lil Native S30V, Benchmade 555HGBlu, Ontario Rat 2 Green G10
On-Deck - Para 3 Cruwear, Sage 5 FRN, Sage 1, Brouwer, Salt LC200N
Wishing-For - Lil Native compression 20CV/M390
Past Knives - Byrd Crow 2 in BD1, Manix 2 S110V, Cat G10 BD1, Delica 4 Blue, Manix 2 S30V, PM2 S35VN