Because it would be a factually based discussion. Here is the EU's list of the major treaties governing it.Why don't remainers want to talk about the future of the EU?
Overview of the most important EU treaties (founding treaties, treaty amendments, Lisbon Treaty, Schengen Agreement, etc.)
You could then ask the question (a) "what powers have been transferred from sovereign states the the EU" and (b) "what powers have been transferred back"?
The answers would be (a) lots and (b) none.
You could also point them to multiple economic papers on the future of the euro - the example below was authored by a Nobel prize winner in economics: Joseph Stiglitz
(Who vastly academically outranks Lance Sapper Crusty PhD)
Which they won't like discussing because papers on the euro, whether they are written by pro or anti-EU authors, are unanimous in saying that for the euro to function, large amounts of further sovereignty must be transferred to Brussels, including the right for Brussels to set member states budgets, fine them it if they fail to meet the budget and organise a transfer of wealth from richer to poorer regions. (The latter is already being done by debt mutualisation - something now being challenged by the German Constitutional Court as being in breach of EU law).Europe has not done well in the years since the 2008 crisis, with a double dip recession and a recovery far slower than that of the US from whence the crisis came. Among the key reasons for this dismal performance is the euro, or more precisely, the structure of the Eurozone, the institutions, rules, and regulations that were created to ensure growth and stability of a single currency amongst a diverse set of countries—and the failure to do some of the things (like the establishment of a common deposit insurance system) that should have been done. The paper describes how Europe created a divergent system, with increasing disparities between the richer and poorer countries, and the role of certain beliefs, prevalent at the time, but since questioned, about what makes for good economic performance.
Remainers can't afford a fact based discussion because the undeniable facts stated above are damning and impossible to explain away by a reasoned and intellectually coherent case.