I feel like the mental stumbling block that stops people has something to do with our ideas of purity and cleanliness. Maybe people intuitively feel contact with the old system would make the new code unclean.

It is slow and painful and requires you to pay lots and lots of careful attention to the behavior of the existing enterprise system that you may very well hate.

Greenfield development sings a siren song. You get to scribble on a gloriously empty page in your imagination, free of such mundane concerns as cash flow, near-term customer demands, day-to-day stability, vitally important edge cases, and hard-won but crufty bug fixes.

Part of the reason could be technical analysis paralysis. I've worked on projects rewriting PowerBuilder components in C#. Getting the two to talk to each other is non-trivial, so determining where to slice off chunks to rewrite is an anxiety-inducing prospect.