Paul Walter makes a valid point:
The problem is that once we start doing referendums, then we need to go the whole hog and do it like the Swiss.
As the above quote suggests, it would be very usual for the Swiss to have a referendum on the deal to leave the EU, even after having voted initially to leave the EU. It is part of the Swiss process.
I am not fond of referenda but, if you do want to use them as part of the decision-making process, then you need to climb fully on board with the process. Britain voted to leave the EU and the government triggered Article 50 (way too soon, in my opinion) in order to start the process. Now that the withdrawal agreement has been negotiated, it is perfectly reasonable to go back to the electorate to ask whether this is the Brexit they wanted.
Doing so has the additional advantage of actually forcing a decision to be made. Parliament can’t decide, and May has responded by delaying in the hope that something might turn up.
Nothing is going to turn up. The deal is the deal and there is no time to negotiate something new. If nothing changes before the end of March then Britain will crash out of the EU with no deal at all.