Why is it that different economic schools of thought are still taught
There is really on two kinds...
The Nature of Economics
It is well-known that Austrians disagree strongly with other schools of economic thought, such as the Keynesians, the Monetarists, the Public Choicers, Historicists, Institutionalists, and Marxists… The ultimate difference from which all disagreements at the levels of economic theory and economic policy stem… concerns the answer to the very first question that any economist must raise: What is the subject matter of economics, and what kind of propositions are economic theorems?
Mises’s answer is that economics is the science of human action… It is this assessment of economics as an a priori science, a science whose propositions can be given a rigorous logical justification, which distinguishes Austrians, or more precisely Misesians, from all other current economic schools. All the others conceive of economics as an empirical science, as a science like physics, which develops hypotheses that require continual empirical testing.
The views of Mises’s predecessors, Menger, Böhm-Bawerk, and Wieser, are the same: They, too, describe economics as a discipline whose propositions can in contrast to those of the natural sciences be given some ultimate justification.
But Mises by no means merely notices this rather obvious difference between economics and the empirical sciences. He makes us understand the nature of this difference and explains how and why a unique discipline like economics, which teaches something about reality without requiring observations, can possibly exist. It is this achievement of Mises’s which can hardly be overrated.
— Hans Hermann-Hoppe, ESAM.
Now why is this school of thought (what used to be indistinguishable from the orthodox) no longer so mainstream?
Why Academics Hate A priori
University instruction in an a priori science presents special problems if we are to adhere to the principle that an instructor ought to be a researcher as well. In every field there are very few who can make actual contributions to its intellectual treasury. In the a posteriori sciences, however, pioneers and followers work together with the same told, and there exists no outward distinction between them. In his laboratory, every professor of chemistry can compare himself with the great pioneer. Although his contribution may be modest, his research methods are the same.
Things are different in philosophy, in economics, and in a certain sense, in mathematics. If academic positions were contingent upon independent contributions to economics, barely a dozen professors could be found throughout the world. If positions are awarded only to those researchers who had made original contributions, care must be taken to take into account research done in related areas. In effect, this makes the appointment to academic positions dependent upon scholarly activity in other areas: the history of ideas and doctrine, history of economics, and especially the economic history of the most recent past, which is often erroneously labeled economic problems of the present.
The fiction perpetrated in scholarly circles that all professors are equal does not permit professors of economics to be divided into two classes: those who work independently as theorists, and those whose work consists of economic history and description. The inferiority complex of these “empiricists” has led to a campaign against theory.
— Ludwig von Mises, Memoirs
in spite of the fact that we now have data to test their ideas and
philosophies of the economy and see if they really line up with
reality?
Mises’s great insight was that economic reasoning has its foundation in just this understanding of action; and that the status of economics as a sort of applied logic derives from the status of the action-axiom as an a priori-true synthetic proposition.
The laws of exchange, the law of diminishing marginal utility, the Ricardian law of association, the law of price controls, and the quantity theory of money—all the examples of economic propositions which I have mentioned—can be logically derived from this axiom. And this is why it strikes one as ridiculous to think of such propositions as being of the same epistemological type as those of the natural sciences.
To think that they are, and accordingly to require testing for their validation, is like supposing that we had to engage in some fact-finding process without knowing the possible outcome in order to establish the fact that one is indeed an actor. In a word: It is absurd.
— Hans Hermann Hoppe

To use an analogy; it is as if one wanted to establish the theorem of Pythagoras by actually measuring sides and angles of triangles. Just as anyone would have to comment on such an endeavor, mustn’t we say that to think economic propositions would have to be empirically tested is a sign of outright intellectual confusion?
But Mises by no means merely notices this rather obvious difference between economics and the empirical sciences. He makes us understand the nature of this difference and explains how and why a unique discipline like economics, which teaches something about reality without requiring observations, can possibly exist. It is this achievement of Mises’s which can hardly be overrated.
— Hans Hermann Hoppe