why hasn't brexit happened?

Probably the best brief (relatively) summary I've read:

​Many statesmen warned from the outset that British ideas of liberty would not survive a merger with the E.U. The most eloquent early diagnoses came from the Labour Party, not the Tories. That is because the fundamental disposition of the E.U. is to favor technocratic expertise over representative government, and the Tories have not generally been the British party that placed the highest priority on the passions of the masses. In 1962, as Tory Prime Minister Harold Macmillan was eying EEC membership, Labour leader Hugh Gaitskell warned, “[I]t does mean the end of Britain as an independent nation state.... It means the end of a thousand years of history. You may say ‘Let it end’ but, my goodness, it is a decision that needs a little care and thought.”

Gaitskell was right, but it is only in recent years that people have begun to see exactly why he was right. It was always understood that joining the EEC in 1973 compromised Britain’s national sovereignty. All countries that joined had to acknowledge the supremacy of E.U. law over their own. This was a deadly serious thing if you reasoned the consequences to the end. For one thing, it deprived Britain’s monarchy of its (already somewhat vestigial) logic. Monarchs are not underlings: in joining the EEC, Britain could be said to have deposed its queen. Pro-E.U. politicians assured their voters that it wasn’t as serious as that. Britain, they said, had to give a little bit of its sovereignty up in order to receive the benefits of cooperation, the way it did in, say, NATO. Other European countries had done so without wrecking their systems.

But this was a false analogy, as the political scientist Vernon Bogdanor explains persuasively in his recent book, Beyond Brexit. NATO was a treaty. The EEC was a merger. What is more, the EEC that Britain joined had been designed by the major countries of continental Europe in line with their own traditions and interests. It was not in line with Britain’s. Britain had no institutions like the European Commission, an unelected body that could (and still does) initiate legislation. Britain’s politicians didn’t understand the rules intuitively and were less able to work the system. British political institutions were unsuitable as a “farm system” for training E.U. politicians.
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And there was an even larger problem than the loss of national sovereignty, Bogdanor shows. The E.U. destroyed the system of parliamentary sovereignty at the heart of Britain’s constitution. For all its royalist trappings, Britain has traditionally been a much purer representative democracy than the United States, because it excludes courts from reviewing legislation on any grounds. British politicians tried to calm the public with assurances that, where British law and E.U. law clashed, British law would prevail. But the acknowledgement of E.U. legal supremacy in the treaties meant that E.U. law was British law. In the 1980s, British judges began finding that parliamentary laws had been invalidated by later British laws—a normal and time-honored process, except that these new “British” laws had been imported into British statute books not by legislation but by Britain’s commitment to accept laws made on the continent. Bogdanor, who is a Remainer and a defender of human rights, does not necessarily condemn this development. But it meant that, through the back door, judicial review was being introduced into a constitutional culture that had never had it.

Here.