Just a historical note. There once was a time when MTBs, as we know them, didn't exist. In order to create multi geared bikes with large capacity derailleurs, the first builders had to adapt touring gear for their early MTBs. Mountain bike and touring components have always been similar and interchangeable. Touring bikes went out of fashion in the nineties at the same time MTBs became dominent, so there are a lot of MTB components available.