Time.
It's not that they're unaware of the importance of non-destructive editing. But adding it to GIMP required a pretty complete overhaul of the underlying architecture, which ended up taking much more time than initially anticipated. Basically the developers had to choose between completely stopping the development of GIMP and focus all their time re-implementing and re-writing the underlying architecture, or to keep working on GIMP as it was, and to slowly changing the underlying architecture bit by bit as and when they found the time. They chose the latter option.
Non-destructive editing has been in development, on and off, for about 15 years now.