At least here the difference is due to spare parts availability. With Honda spares aren't an issue, albeit their dealership network is shrinking, and you can get spares even for eighteen years old engines no problem.
Subaru/Robin... well, that's another matter completely. These days with the Internet it's far easier to get parts, and in other countries it's probably different, but I've never seen a workshop manual for their engines and their only dealership remotely "local" is over sixty miles from here.
I don't question for a minute Robin makes excellent engines but it seems their business model is the same as many Japanese brands in the 70's: sell the finished product but then forget about parts and assistance.
Now that Toyota has bought a large stake in FHI I hope this will change...