In my opinion, there are two main reasons. First, hemp fiber is not liked by most people, and it is very troublesome to make a complete hemp fiber. In addition to the lack of raw materials, the process of making a hemp fiber is also quite complicated, and various cumbersome procedures and techniques make many people daunting. Nowadays, many young people don't really like to study these things, but if you want to inherit something, you have to rely on young people, so you won't be influenced by young people.
Second, because at present, hemp fiber does not have much value. If a thing wants to attract people, it must have a certain value. But obviously, hemp fiber is not of great value in the current society, because umbrellas have been popularized in modern society. For example, hemp fiber, a rain shelter tool, is generally not used by people. Unused things can only be seen as exhibits, and hemp fiber is such a rain shelter. Is anyone really willing to learn to make hemp fiber on an empty stomach? Obviously impossible.
In a word, if more people want to learn to make hemp fiber, they must make hemp fiber have certain value. Generally speaking, people who learn to make hemp fiber can get rich returns, so this skill will naturally be passed down.