Matrox and 3dfx never came close to making general purpose GPUs, they just got completely outcompeted in the regular PC graphics market of the late 90s. Matrox and 3dfx last tried to compete on architecture in 2002 [1] and 1999 respectively, while the first GPUs capable of general purpose computing were released many years later at the end of 2006 (by Nvidia) and mid 2007 (by AMD/ATI). High performance GPUs have been a completely Nvidia and AMD/ATI affair for two decades until Intel entered the fray last year, since before people even conceived of GPGPUs.

Matrox and 3dfx were gone before they could even conceive of a general purpose GPU strategy.

[1]: The Matrox Parhelia itself being out of date tech on release to boot, lacking a Direct3D 9.0 class architecture that ATI launched two months later.