The short answer to this question is by the Illegal, Unconstitutional and Undemocratic actions of the democratically elected Politicians of the UK and the Illegal and Undemocratic actions of the EEC between 1961 and 1971.
The Illegal, Unconstitutional and Undemocratic actions of the democratically elected Politicians of the UK
This was with a campaign over more than a decade of lies, deceit, manipulation, brainwashing, Government sponsored propaganda, business and media sponsored propaganda, illegal actions such as distributing a favourably edited version of a White Paper that had not received Parliamentary approval and using Civil Servants to do it, the repeated denial of the People’s Democratic Right to a Referendum or General Election based on whether or NOT to join the EEC/ ‘Common Market’ and ignoring all Public Opinion of up to 84% against joining the EEC.
All three main Political Parties of Conservative, Labour and Liberal were led in by obsessive Pro-Europe leaders and all dissenting back bench MPs were belittled and ignored. Edward Heath as Prime Minister ignored the fact that he did not have a Legal Mandate from the People to join the EEC and bribed, manipulated and coerced MPs to back him in Parliamentary Votes (notably on 28/10/71) to join the EEC by force in the manner of a corrupt Dictator.
The Illegal and Undemocratic Actions of the EEC
The EEC (‘Common Market’) exploited their advantageous situation to the maximum whilst knowing full well that the People of the UK by an overwhelming majority did not want to join. Under those circumstances and knowing that the People of the UK desperately wanted a Referendum but were being denied one by the UK Government they should have DEMANDED that the UK Government carried out a Referendum (which is a well-established continental practice of consulting the People and did happen in 1971 on this exact issue for the People of Norway, Ireland and Denmark) as one of their many terms of entry, on the straight question of yes or no whether the People of the UK wanted to join the EEC.
We assert that in not doing this they acted in contravention of International Law and Undemocratically to the point of the suppression of the Democratic rights of People of the UK. They did not want to demand a Referendum as they were purely motivated by the money and World influence the UK would bring to the EEC and they knew that the People of the UK would overwhelmingly vote AGAINST joining. This is in direct collusion with the UK Government who denied a UK Referendum on entry for this exact same reason.
They also took further advantage as they demanded punishing entry terms such as extortive financial contributions, forcing capitulation terms for agriculture and fisheries, having to comply with copious legislation, rules and regulations, having to comply with free movement of labour, services and capital and taking away many of our rights such as free trade and the abandonment of the Commonwealth. All against the will of the People and without the People of the UK being allowed to have their say and a Democratic Vote!
The EEC let Edward Heath sign the Treaty of Rome in 1972 knowing full well he did NOT have the consent of the People of the UK to do so and by doing this let the UK join the EEC Illegally, Unconstitutionally and Undemocratically.