Because it's one of the strongest arguments against capitalism (state-sponsored usury) in a magisterial docuмent.
If it's "state sponsored," then it's not capitalism. Distributism is simply ideal capitalism/free enterprise.
Predatory usury, central banks, monopolies, unprofitable activities, etc can only happen precisely because of government intervention, aka socialism. 99% of countries in the world run the same economic system that Hitler's Germany ran, and a similar system was advocated by Keynes. Keynesianism and/or the Bretton Woods I and II systems are basically a modification of national socialism with less racialism.
Every country is doing "state capitalism," which is exactly what the Germans were doing and what Stalin ended up doing anyway with "socialism in one country." There has never been a truly free enterprise system, simply a matter of lesser or greater socialism.