Why do child sex abuse cases get dropped for "lack of evidence"

Times article takes the right tack about Panorama programme on Jimmy Savile, about the chance missed for him to face justice when he was alive. The BBC Newsnight editor seems to have decided not to broadcast their programme because initially he thought the case had been dropped because Savile was too old, but in fact it was because there was said not to be sufficient evidence to prosecute. There is an issue about how commonly the CPS drop such abuse cases that Panorama did not pursue in its programme, instead implying more about the pressure the BBC was under not to create a clash with its Savile tribute programmes. Ed Miliband was right to call for an independent inquiry (see video) which could deal with the wider societal issues.