Because in large part it is absolute nonsense.
This is a very technical problem, but in the sense that you should be able to produce an individual, operational, description of any economic phenomenon in order to assert a truth claim ( trace the incentives), that’s true.
In the sense that this technique is sufficient for the identification of all economic phenomenon it’s simply false – and we have many cases now.
In other words, Mises tried, like Marx and Freud and Boaz, and Cantor to create a pseudoscience. And he failed. And he has been justifiably ignored because of it.
You have also been the victim of propaganda as well, since it was very clear at the time, as it is very clear to any serious student of intellectual history, that there was a vast difference between the Mengerian (Polish-christian) wing, and the Misesian(Ukrainian-Jewish) wing of the Vienna school.
And the findings of the Mengerian school (a) caused marx to stop writing about communism, (yep, really), and (b) have been fully incorporated into contemporary economics under the terms “mengerian”, “marginalism”, and ‘subjective value’.
If you want to put these movements in context, here is the difference between the major schools of economic thought (The conflict between rule by discretion and rule by natural law).
The Propertarian Institute.