The referendum will be decided by 35% of the population who don't really care either way and will vote on the recommendation of the politicians they trust, which in 80% of referendums around the world those politicians are in the government they just elected. This referendum is likely to conform to that pattern* and those who want to leave won't accept the result anyway;
"It was a government/establishment stitch-up," "they didn't campaign on the real issues." And the first piece of social or environmental pan-European legislation post-referendum will also produce cat calls from the likes of Farage who will tell you that "this is not what the British people voted on they voted for free trade and we've been conned again."
Belonging to the EU is not some sideshow but a fundamental issue about where you want to take the country. Any leader who calls a referendum on this issue is masking the fact that they are not in control of their own party. It's lucky for Cameron that most of the public don't care enough about the EU either way to question this weakness.
* Although Toby Young on Newsnight made the point that this referendum will be different to 1975 as the inclusion of Michael Gove and Boris Johnson will mean that Out campaigners can't be portrayed as cranks and chancers.