He has conveniently forgotten it seems that he failed miserably in negotiating a better deal with the EU, and this led to his promised referendum
I think the fact that the EU ignored Cameron, the PM of the UK, and made NO concessions (we weren't asking for much) and dismissed him, made people more and more concerned that getting out of the EU was in our best interests.
Our EU membership had been a topic of political conversation for a long time.
Please remember, not just the Conservative Party, but the Lib Dems and Labour were also listening to the mood of the UK and involved in matters of getting out of the EU and holding a referendum.
Our EU membership had been on the agenda and a big bone of contention for a long, long time before the 2015 elections.
Labour for a Referendum was a political campaign by members of the Labour Party that sought a referendum in the United Kingdom on the European Union. The movement was set up following a pledge by the Conservative Party to hold an in–out vote if re-elected in 2015 United Kingdom general election
Following the Conservative victory in the 2015 election, the Labour Party committed to supporting passage of a Referendum Bill through Parliament
And as for Clegg, then leader of the Lib Dems, here's an inconvenient truth: He previously supported an in/out EU referendum.
In their 2010 election manifesto, the Lib Dems called for a national vote on Britain's EU membership
Here's the pledge in full:
The European Union has evolved significantly since the last public vote on membership over thirty years ago. Liberal Democrats therefore remain committed to an in / out referendum the next time a British government signs up for fundamental change in the relationship between the UK and the EU.
Brexit was on the cards long before 2015 and it was a cross party issue.