It takes too long. The algorithm is computed much faster through the architecture of a CPU.

A GPU could technically do it, but at a much slower rate for a much higher electricity cost.

This goes beyond my full understanding, but it comes down to math. The algorithm uses a type of formula and execution method that is carried out much faster by a CPU. To get more info you need to read into technical pamphlets that require lots of background knowledge or willingness to learn it