Its not up to the government to clean up sport, its up to the organisations that run the sports to clean it up. Its only when criminal investigations come into play on the scale of FIFA for example that a government feels compelled to get involved. They cannot also get involved based on speculation, it has to have some substance to it i.e. evidence which invariably comes from media/whistleblowers. I'm pretty sure everyone can say with some certainty that drugs are used in boxing, cycling, athletics, football etc. but its up to their organisations to sort that out along with independent bodies such as WADA.
If Cameron said tomorrow, right I am stumping up £20m to investigate drug taking in boxing and stamp it out, you would have most of the nation up in arms saying that its a waste of money when people are queuing at food banks etc. as well as the likes of WADA feeling somewhat aggrieved and asking why the government are intervening on their patch based on speculation.