My guess is: 'brute force and fast' always wins against 'elegant but slow'. And both the 3dfx products and triangle rasterization in general were 'brute force and fast'. Early 3D accelerator cards of different vendors were full of such weird ideas to differentiate themselves from the competitors, thankfully all went the way of the Dodo (because for game devs it was a PITA to support such non-standard features).
Another reason might have been: early 3D games usually implemented a software rasterization fallback. Much easier and faster to do for triangles than nurbs.