3dfx killed 3dfx. They bought out a board partner in an attempt to become vertically integrated. That unnecessary distraction quickly put them 1-2 generations behind Nvidia and ate away a chunk of balance sheet even while their other former, now-aggravated board partners began competing to release a pile of Nvidia boards.
3Dfx saw competition coming and went for a radical option, merging with a loss-making OEM (STB) with an intention to shake things up, which backfired hugely.
3Dfx had 25 or so board OEMs as customers, and basically told them all to "f-off and stop giving us money for our product, we're going to compete with you". At a time when there was starting to be competition for thoe OEM customers from newbies NVIDIA, and ATi was having to step up its game too. 3Dfx had ony been around a couple of years too, so it's not like it was entrenched (despite what fanbois said at the time).
STB was one of NVIDIA's biggest OEMs, the exclusive launch partner for the Riva TNT (
) and with contracts to put its NVIDIA boards in PCs from Gateway &c. So 3Dfx not only took its GPUs away from ~25 graphics board OEMs, it took a major board OEM away from NVIDIA. Who could possibly have predicted what would happen next?
Add in that 3dfx+STB wasn't very good at getting product out to market on time, so its Voodoo4 didn't arrive when it would have been okay, and that was the collapse ... and management's response to that was to merge with Gigapixel, to address "a number of issues in the area of silicon execution" = bankruptcy in 2.5 years from a market-leading position.