Look in Hybrid and Gravel forums. There are lots of conversion bikes. I just ran across a thread in Classic & Vintage about drop bar conversions on MTB's last night. Quite a few pictures.

I'm sure it's all over the country, and the MFG's are just following what the public is doing and trying to sell something ...

As I see it, the main difference is in the seat stays and the chain stays, and maybe fork rake... The MTB's all have longer chain stays so they can be ridden up steep slopes. Road bikes suck the wheel right up to the seatpost and would be unable to climb as steeply. Fork rake is called slack in MTB world, but it's still about how lively the front end is. Forks w/ or w/o offset axles can fix some of this in short order if you wish

There are some inspiring build-outs in this thread: Multi-Track