Why is there an enormous difference between SAT solvers?
Yes, there is a major difference between MiniSAT and WalkSAT. First, let's clarify - MiniSAT is a specific implementation of the generic class of DPLL/CDCL algorithms which use backtracking and clause learning, whereas WalkSAT is the general name for an algorithm which alternates between greedy steps and random steps.

In general DPLL/CDCL is much faster on structured SAT instances while WalkSAT is faster on random k-SAT. Industrial and applied SAT instances tend to have a lot of structure, so DPLL/CDCL is dominant in most modern SAT solvers. Instance to instance one technique may win out, though, which is one reason why portfolio solvers have become popular.

I take a lot of issue with your claim that WalkSAT is much faster than MiniSAT on the instances on that page. For one thing, there are gigabytes of SAT instances there - how many did you try comparing them on? WalkSAT is not at all competitive on most structured instances which is why it's not often seen in competitions.

On a side note - Vijay is right that MiniSAT is still relevant. Actually, because it's open source and well-written, MiniSAT is the solver to beat in order to show that a given optimization has promise. Many people tweak MiniSAT itself to showcase their optimizations - take a look at the "MiniSAT hack" category in the recent SAT competitions.