Citizens voted in a non-binding referendum that the British parliament was/is under zero constitutional or legal compulsion to listen to.
Let's be clear about that -- the sovereignty of the UK rests with parliament, not with the British people, this is not the USA -- the parliament could have legally completely ignored the results of the referendum
Next, citizens voted to leave the EU but were not clear about or didn't think about:
- free trade - that would end unless the UK joined the EFTA - basically pay, follow the rules and have no say in the decision making process (Iceland/Norway/Switzerland) - oh and still have to let EU citizens travel and work to the UK
- The NI-RoI border - the UK had signed in the 90s the Good Friday deal to keep the border open. How to relate that to closed borders with the EU was unknown and is still unknown