Because huge numbers of British people feel they have been cheated by all three major parties on this issue.
Thousands of laws emanating from Brussels have been passed by the British Goverment, often without debate. How can we expect to be law-abiding citizens without knowing just what laws exist?
A prime example of bad laws accepted by the UK is the Common Fisheries Policy, which has resulted in 'illegal' food being thrown back into the sea, loss of jobs in the industry, and foreign fishing vessels being allowed to fish in our waters while British fishermen go hungry.
Other examples of the way that EU law affects our everyday lives and makes us all either potential criminals or unemployed are the EU rules on subsidising post offices (causing many to close) and on landfill (resulting in the ridiculous bin offences).