Probably the biggest reason the Austrian School is "fringe" (a fairer descriptor is "heterodox") is that there's no money in it.
Economics is a value free science, at least how Austrians practice it, but happens to show that government intervention is usually harmful. For example, Keynesians believe that the business cycle is an inherent failure of markets with no known cause and that government must intervene heavily to correct such errors. By contrast, the Austrian Theory of the Business Cycle takes nearly the opposite position -- that intervention, mainly in form of credit expansion causes booms. A bust is a correction of errors made during the boom and should not itself be corrected with more easy money, starting the cycle all over again.
Now, the vast majority of working professional economists derive all or much of their income from the government in one way or another. Many work for the federal government or the Federal Reserve Bank or consult with them. Or they work in government funded universities doing research with money from government grants.
Early last century, Hoover then FDR discovered and embraced John Maynard Keynes who offered a general theory that supported heavy government intervention. The Keynesian prescription just happened to provide an intellectual basis for policies that would require government to grow much larger and more powerful. Before long government began to fund more and more professional economist jobs. And no surprise, those jobs went to Keynesians.
A few decades later Milton Friedman (not an Austrian) said, "We are all Keynesians now" -- not as an admission that the theories were correct but a concession that in practical terms it's nearly impossible to work in the field and not be a Keynesian.