Why is there an enormous difference between SAT solvers? SAT solvers are very important in algebraic attacks, for example walksat and minisat.

However, when solving the benchmark problems available here there is an enormous performance difference between the two - Walksat is much faster than minisat for these problems. Why is this?

This implementation of walksat appears to have some performance improvements - is there any reason it wasn't included in the international SAT Competitions?
SAT solvers don't have a single approach -- not every approach works equally well for every problem. A SAT solver that performs well on a given problem will not necessarily work well in general.

Typically SAT solvers employ a whole bag of tricks -- deciding what heuristic to employ is one of the big differences between solvers. Different heuristics work well for different types of problems and a solver must identify traits of the problem at hand to decide which heuristic to use (unless the user specifies which heuristic to use, which one might, if one knows what to do, or for experimentation purposes).

As to why a given solver wasn't entered in the international SAT competition, you'd have to ask the creator of the solver! I'm pretty sure they could submit it if they wanted to...