I completely agree that more arguments should have been made about the positives of the EU but I think by the time the referendum was called the die was already cast. Governments of both stripes were guilty of painting the EU as either overbearing, incompetent or irrelevant by turns in order to take credit for EU successes and excuse their own failures. The two big issues for Leavers, the perception that the UK had lost its sovereignty because of EU regulation and that was suffering due to unfettered immigration, were egged-on for political gain by both parties throughout the 2000s. Some of the loudest Remainers during the referendum and to this day are the people most responsible for the UKIP-ification of the debate surrounding migration. Blair's contribution to that was huge (I posted about some of it
here), he outflanked Ian Duncan Smith's Tory party from the right on immigration in the early 2000s and his offer to the nation prior to the 2005 election included a points-based immigration system and a referendum on whether to leave the EU. Cameron spent 6 years setting arbitrary immigration targets he had no intention or ability to hit and and then blaming it on the EU, Labour under Milliband were selling 'Controls on immigration' mugs going into the 2015 election. When many of the same politicians turned round in 2016 and tried to desperately shore up the dyke they themselves had spend decades tearing down they looked ridiculous. All of which is to say, by the time the referendum was called I don't think many people were legitimately on the fence about it. When it came to the polls, the actions of party leaders during the campaign was probably the last thing on peoples' minds when they voted. In fact, the fact that the politicians didn't want it to happen was a huge plus for Brexit in many peoples' estimations.
In terms of post-referendum, the way Labour played their cards on Brexit was one of the reasons they pulled off a bit of a shock in 2017. I think I'm right in saying that the stats and polling backs up that had Labour gone full Remain (ref. on deal/2nd referendum/cancel Article 50 etc.) into that election they would have been absolutely massacred. I think Corbyn sitting on the fence re: Brexit following that election was part of the strategy to keep the electoral coalition together in case we saw another one. But there's been no realistic chance of that happening for ages now, and like I said, both sides of his coalition are getting understandably tetchy. For my money Labour should have taken the plunge into supporting a referendum on the deal (at the least), about 12 months ago, not because it was politically expedient or to stick it to Theresa May, but simply because it's in the country's best interest. Whether it's a lack of boldness from party leadership or whether it's just that those at the top are quite happy with the soft-Brexit Corbyn is angling for, I'm not 100% sure. Either way it's fecking annoying.