From memory, it was that Keith Packard felt that the bits of the original server that run on the CPU (mfb, cfb) were optimized for CPUs without caches, they unroll the various bit operations and generate C functions for each one which increases code size.

One of the first targets for KDrive was the Compaq iPaq with a StrongARM CPU and no GPU.

I think the ideas behind KDrive have now been folded into the main X.Org server.