>>1672300Main reason is the lack of compatibility between road and mtb stuff - for the most part. Which makes mix-matching stuff kind of a chore that very few people would do. Most just want a full-package bike without the need to faff around.
Up until recently getting disc brakes that could work with brifters was difficult and 10 years ago it was pretty much impossible.
With the increasing popularity of gravel bikes - you will see more drop-bar mtbs as well.