I'll bite, since this is something pops up, and is repeated "why don't MMAers use it, why don't <insert whatever practical ass whupping MA> use it"
Without going into the technical details, transmitting the full-blown "how" to train these skills is hard enough. I'm guessing it was really meant to be, and was transmitted in small teacher:student ratios.
It requires a lot of work on both the teacher and student. Think of it like a Professor vs. pet student. Said Prof may pick several students as being likely candidates to pursue a given subject deeper, and gives them personal instruction. The rest are given a gloss-over by-the-book version.
For most subjects, you can pick some stuff up even in the "by-the-book" version.
Unfortunately for IT, even for the average stuff, it needs to be shown hands on, in order to get your foot in the door.
It's also hard to rewire everything. It's not like, "do this" and you suddenly turn the switch on. For example, you can tell a karate guy to kick the internal way step by step, and he'll have a hard time doing it because he's too used to kick in a certain way. He would have to go home and think, retool, and practice at a boring slow rate to get it.
Also, it takes a lot of thinking and pondering--at least for me-- to do this stuff. You can't just go and mindlessly pound some bags. You have to think about how to do it, set up the 'intention', make sure not to do certain things, etc. etc.
This can take a few years (not many years) to rewire everything depending on the student and coach. But that could be too long for a lot of people who are already good at fighting or need to learn how to fight in a short amount of time.