Although our voting system is very flawed, so that a party can gain a majority of MPs by a getting a minority of votes, we still have a representative democracy. MPs are charged with exercising their considered judgement on every issue. They should vote according to their conscience. They are not delegates who must follow the instructions of their constituents.
Most MPs, who have much better understanding of politics than the average person, came to the conclusion that the best interests of the UK would be best served by remaining in the EU.
There was an advisory referendum in which the leave side got slightly more votes than the remain side and MPs had a duty to take that into account, but they knew there were many factors influencing that decision.
Those MPs who still believed that we should remain but were bullied by their party whips or feared for their careers, and acquiecsed in supporting brexit totally against their better judgement let our country down.