One very plausible theory I've seen doing the rounds is that the government never actually expected to have to follow through on that campaign promise. Strategically speaking it was a terrible idea and Cameron was always pretty pro-EU, but he needed the support of the anti-EU faction of his party, so he promised the vote as a sop to them. The theory goes that he didn't expect to get an absolute majority in Parliament and hoped the other parties would "force" him to to not follow through.