Because the 1975 vote was not solely on the situation at the time. It was about being part of whatever changes were made, without needing a new referendum on staying in each time.
Edited to add, principally for the hard of thinking (such as the button-bashing incumbent SPotY):-
"Britain's membership of the European Community presented a “gross infringement of sovereignty” and a “serious attack on Parliamentary democracy,” Cabinet ministers warned ahead of the only referendum on Brussels given to the public, newly uncovered papers reveal. Harold Wilson, then the Labour Prime Minister, was told choosing to stay in Europe would represent a “dismemberment of the authority of the House of Commons”, minutes of a Cabinet meeting show. In a meeting three months before the 1975 referendum, Mr Wilson was urged by his ministers to inform the British people that membership would seriously compromise Britain’s ability to govern itself. In the event, the Government’s official pamphlet explaining the referendum gave no such warning – and instead assured voters that the “essence of sovereignty” would be protected by staying in."
"Mr Wilson urged voters to vote Yes, and claimed to have won a “new deal” for taxpayers after a “long and tough” renegotiation battle. Historians now regard those changes, mostly to the terms of agricultural trade, to have been little more than cosmetic – but the Government’s case was enough to swing voters behind staying in by a margin of two to one. However, newly uncovered minutes of a Cabinet meeting in March 1975 showed how ministers argued the electorate should be warned ahead of the referendum that directly applicable laws are an “essential part” the fledging European bloc. Their impact “should be made clear to the electorate” before the vote, “enabling a decision on this issue to be taken through the medium of the referendum”, the minutes show. Edward Short, the Lord President of the Council, told colleagues that while the Commons could always vote to withdraw from Europe and the UK retained a veto on the Council of Ministers in Europe, it was 'essential not to obscure the fact that Parliament’s control of day-to-day legislation had been diminished by accession to the community.'”
How Harold Wilson was warned Europe threatened British democracy
"The newly released documents show that Cabinet ministers Michael Foot, Tony Benn and Peter Shore wrote to Wilson in November 1974 about the planned vote. They said some ministers would support continued membership whatever the results of the negotiation. Others - meaning themselves - might feel that 'however great the improvement of the terms may be, the loss of sovereignty implicit in membership would involve paying too high a price.'"
BBC NEWS | UK | UK Politics | How Wilson handled his Euro split
"Secret Cabinet documents released from Whitehall this week gave a fascinating insight into the way voters were duped in the run-up to the 1975 referendum on Britain’s links with Europe. The Foreign Office papers were declassified following a year-long freedom of information battle by the Eurosceptic pressure group Britain for Business. And they reveal that official advice given to ministers in Harold Wilson’s Labour government about the full impact on the country of joining the European Economic Community, the forerunner to today’s European Union, was not disclosed to the public.
Ministers were warned that the lawmaking powers of the Brussels bureaucracy would lead to “a gross infringement of the sovereignty” of the Westminster Parliament.And a senior official told Mr Wilson, who was then prime minister, that transferring powers to the Brussels-based European Commission threatened the 'most serious attack on Parliamentary democracy with which this country was faced.'”
Heath's EEC Deception