One basic reason why politics has reached this point is that members of parliament were historically poorly paid for the amount of work that modern MPs expect to do. In 1983, when Gordon Brown first went to the Commons, an MP earned just over £15,000. It was an absurdly low figure even then. So what did those who could have changed the system do? They did nothing. Margaret Thatcher refused to give MPs the increase they needed or the framework for future salary review that would have put parliamentary financing on a defensible basis. And John Major, Tony Blair and Brown all followed her lead. Today's £65,000 parliamentary salary is better in real terms than 1983, and it is certainly a good income, but it is not high when compared with legislators in many other countries, or with the professions with whom MPs might sensibly be compared.
Why did Thatcher and the rest hold off? Not because MPs didn't need the money or wouldn't vote for it. They held off because they were afraid of the newspapers, particularly the Sun. They were not prepared to risk the wrath of Rupert. It was the press who stood between MPs and a sensible income. So the true patron of the expenses system against which the press rages today is the press itself.
To see the Daily Mail, owned by the tax-dodging Lord Rothermere (Private Eyes, passim); and the Daily Telegraph, owned by the really weird Barclay brothers, lead an effort at lynching is disgraceful.
UPDATE: One of the worst cases so far has been of the married Tory MPs Andrew Mackay and Julie Kirkbride. But in fact this story was, to its credit, pushed by an internal Conservative Party review. The Telegraph said nothing, perhaps because Ms. Kirkbride was a senior former employee there. See article at The Guardian.