Well -- they didn't.
Their first creation, the "NV1", wasn't successful and used a weird way of accelerating 3D graphics (not polygone-based) ... so they made a break. Later, the "Riva 128" a.k.a. NV3 could compete with 3dfx' Voodoo Graphics in Direct3D -- but of course not in Glide, but it was a jump. The "TNT" in 1998 was the first nVIDIA creation that could beat a 3dfx card if you look at the features. 32 Bit, 2048x2048 textures. And this thing was as fast as a Voodoo2.
Then, in 1999, with the TNT2 it became worse. The "Ultra" version was often faster than a Voodoo3 3000 and the later released GeForce256 kicked everything with it's speed and hype (T&L). Just in Glide games the V3 3500 was faster.
In 2000 - we alle know this year as a bad one for 3dfx fans - nVIDIA threw their GeForce2 series on the market. Being a tuned GF256 it was damn fast -- faster than 3dfx' answer ... the Voodoo5 5500.
Combined with a series of management mistakes 3dfx went down and nVIDIA bought all relevant brands etc. for a dumping price. You could see this positive: If they didn't, many 3dfx engineers would't be employed now ...
Just a short-version of the story. Look in the internet, there are many articles about this topic.
Greetings,
Raff