I'm working on such a re-write / learning process right now because underlying stuff has fundamentally changed and it was (slightly) long past time to make the move. Of course, we're still going things incrementally and slowly and making effort to keep older tech running.

For some tech choices there was just one clear path from where we are to where we need to go. But for others, the overriding decision was more often than not that some team member had familiarity with the technology. Certainly not the only criteria but enough to tip the scales if it came to that.