Concrete contingency plans with third countries would have been strictly worse
Part of the Brexit sales pitch was that the UK could have achieved better deals outside the EU. This turns out to be false, and indeed part of the point of being in the EU is to secure better deals.
Some attempts were made, but with no real success. India were not in a hurry. India wanted more migration access to go with any trade deal, and this was obviously a non-starter in today's UK.
No negotiation before notification
The EU simply refused to enter into such negotiation on grounds of principle.
No clear mandate
Prepare what, exactly? There have always been multiple conflicting "visions" of how Brexit should be done.
The whole thing was directly opposite to the way in which the Scottish Indyref was carried out, in which there was the SNP in power in the Scottish Parliament with a large clear manifesto on how it should be done. If that had been a success it was fairly clear what the plan was and who would be carrying it out.
The same was not true of Leave. Most of the key campaigners have never been in elected roles. UKIP have only ever had a couple of short-lived MPs. Moreover, there was no surge to UKIP in the 2017 snap election. Not only that, there were two "independent" Leave campaigns (although this may turn out to have been an attempt to cheat spending rules).
The Three Brexiteers
What a lot of people were expecting was that one of Gove, Davis, or Johnson would have taken over in the post-Cameron leadership election. They did not enter as candidates. This left May, a Remainer, to implement a plan she had no support for.
Tory Syriza
You cannot hold a referendum that obliges other countries to give you things.
Greece tried this at terrible cost and was ultimately unsuccessful. If there is a country with a case for leaving the EU, it is Greece, not the UK; but ultimately at the crunch time they decided they would have been worse off out.