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?
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Walksat is an incomplete solver. This means that it tries to find a solution for a number of iterations. If it does find a solution it answers with the solution, otherwise it answers "don't know". Walksat uses a form of random walk to search for solutions with heuristics to guide its step.
Minisat on the other hand, given enough computational resources, gives you either a solution or answer that the problem is unsatisfiable (and by adding a proof-trace you also get a proof). Minsat uses a modern form of DPLL.
The short answer to your question is that it is easier to search for an answer, if it exists, than proving than no answer exists.
The longer answer is that walksat is only faster than minisat if there is a lot of solutions and their structure is of a special kind in the search space.
On the page with benchmarks I could not find any reference to walksat. Have you testet walksat on some benchmarks yourself? Have you tested walksat on benchmarks that are known to be unsatisfiable?
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...