Basically because Southwest Airlines were the launch customer and also the single biggest customer of the B737NG they had a huge input into the design of the cockpit and cabin. The overhead is almost unchanged because SWA operates B737-200s through to -700s (I believe almost 350 total) and to maintain the single type endorsement the overhead was modified but basically unchanged and the PFD/ND display was "dumbed down" to look like the staem powered gauges in the -200.
This is what I have been told by our company reps.