That is a very interesting question. Yes, it is true that the policies advocated by Keynes and the Keynesians benefits those in power. After all, it is increased expenditure on the part of the government that is supposed to be the elixir that lifts the economy out of a chronic recession. Which politician wouldn’t love that piece of advice?!
Nevertheless, it would be wrong to conclude that Keynesianism is popular only because of political propaganda. Most economists who espouse Keynesianism do so because they sincerely believe that those policy conclusions will help economies recover from recession and thus benefit people who have lost their jobs and are suffering financially.
So if you are looking for the reason that the vision of Keynes is so popular, you must look beyond vested interests and study the underlying economic theory. It is essential to understand the underlying Keynesian theoretical framework, isolate the flaws in it and then try to investigate why economic theorists are swayed by those faulty arguments. It is ultimately because the ideas of Keynes have won the battle over the ideas of his critics that the doctrines espoused by him and his followers are so popular.