I think this question confuses two different scenarios.
First we should point out that a British Prime Minister didn’t take us into the European Economic Community (as it was in 1973) without any Parliamentary debate. There were actually numerous debates and votes before we joined, starting with a vote of 356 to 244 on the principle of membership in October 1971. If the majorities in favour of the terms of membership were sometimes slender, that’s only because Labour imposed a three-line whip; if it had been a free vote the majorities would have been a lot more convincing. So MPs did debate us joining the European community, exhaustively so. The 1975 Referendum wasn’t about joining, it was about whether we should stay in, and we voted 67:33 to remain.
It’s ironic that in 2016 we have been very close to reversing that decision without a Parliamentary debate. That’s what Theresa May proposed, although she seems to have backtracked somewhat. Some people apparently voted Leave in the EU Referendum to uphold the principle of Parliamentary sovereignty. Yet until recently Mrs May was contemplating a move that itself would have made a mockery of that very sovereignty. She’s still not allowing proper parliamentary scrutiny and she may still activate Article 50 without a vote in Parliament. So you could better express the question as: “If MPs were able to give their opinions when we joined the European community, why did the Prime Minister intend to ignore them now that we’re leaving?”
There are many complex constitutional questions around this and it’s not surprising that people find them confusing. If, after whatever debates take place, the Government were to use the Royal Prerogative to pull off a hard Brexit without allowing MPs a final vote on it, it might be popular with the tabloids and the most enthusiastic Brexiteers – but it sets a very difficult precedent. In obedience to the anti-elitist, direct democracy of the Referendum result, Mrs May wants to use one of the very oldest, most elitist instruments in the constitution: the Royal Prerogative. That’s doubly ironic.
And the precedent would be quite troubling. Our membership of the EU is one of the most significant international agreements we’ve ever made. It’s comparable to our membership of NATO or the UN. Imagine if, say, a socialist Prime Minister with a very slender Parliamentary majority wanted to withdraw Britain from NATO using the Royal Prerogative rather than giving the final decision to Parliament. Would people be happy about that?
Referendums are notorious for their tendency to inflame passions that cloud the argument. That’s why we have representative democracy. And the lesson of the Brexit vote is, you should only call a referendum when you’re sure what the answer will be. Cameron seemed to think that since the AV Referendum and the Scottish Independence Referendum had gone his way, this one would too. Instead it was third time unlucky, landing us in a situation full of constitutional anomalies, whatever the economic consequences might be.