In my opinion, 3dfx made the mistake of "wanting their cake and eating it too".
1) After the release of the Voodoo2 chipset (at that time they did not sell video cards only chips), the stocks were sky-rocketing! However, they decided to produce the end product video card and not sell to other companies. This killed a lot of competition for their own cards, because they were the sole makers.
2) Plus, I believe when 3dfx started to make 2d/3d cards they lost something. I remember when you had a 2d card (normal computer users), but gamers had an additional 3D video card!
Anyway, that's my personal spin on 3dfx.