Initially Britain did not join the EEC because it was not seen as desirable given the fact that Britain at the time that the EEC was being founded still believed that the empire could provide a better market. Even with the Suez crisis Britain was too late to become a founding member. Britain thus 'missed the bus', explaining one reason why we took so long to join the EEC.
The reason that Britain did take so long to join the EEC after this point was primarily due to De Gaulle. Britain had two unsuccessful attempts to join the EEC, both of which failed due to De Gaulle famously saying 'non' to these applications. When we successfully did get accepted in 1973 De Gaulle was no longer in office. This would further explain why Britain took so long to join the EEC