I've posted about this for years and it has less to do with effectiveness and more to do with the pressure on a coach to produce quick results for parents. The 2hbh is just easier to teach. It allows for quicker results in timing, racket face alignment, stability, and control, where a 1hbh takes longer to coordinate those check points. And as mentioned above, most coaches like to replicate their own strokes and never took the time to learn a 1hbh themselves, so teaching it only allows them talking points over experience and being able to demo/model the stroke for clients.
Reminds me of ye 'ol Betamax vs. VHS story, and how/why VHS won out.