A vote was held, and the decision was to leave. That’s fine - we should not have run that referendum in the form it was, but we can’t deny the result.
The problem is, Brexit was an entirely empty vessel into which voters’ fantasies were poured. The outcome in no way resembles any of the ideas that were articulated before the referendum. There is now a concrete plan that basically everybody hates, because it’s the worst of both worlds.
There are three realistically possible options right now. The UK either exits with the agreed deal, exits with no deal in a disorderly fashion, or cancels Brexit and remains in the EU. The first option is bad because nobody wants that deal. The second is bad because it would be obviously catastrophic. The third is terrible because it directly contravenes the outcome of the referendum.
In short, all the options are fucking awful. There’s no real democratic legitimacy for any of them. Under these circumstances, having a second referendum that says “this is what we were able to negotiate, now do you want to go through with it or forget the whole thing” is possibly the least shit option. I hope it will be cancelled, but at least if it’s not I can be sure that it was voted for with knowledge of what was to come.