Because its become entrenched, tribal, not rational. If it was a rational argument between 2 clear economic options we'd never have left. I'd suspect you would have a lot more Leavers abstaining now tho. These voters are still not happy with the status quo, and want change, but maybe they appreciate the problems associated with leaving the customs union more.
The next step is for a bright politician to deliver some of the sentiments of Brexit here in the UK, while staying in the EU. Corbyn tantalises but fails. Maybe he'll come good, but in this climate with the government in pieces and strong opposition leader would walk it. He should be well ahead in the polls, but he's weak, unbelievable, and associated with the past. Which is a shame as some of his ideas are solid. He lacks the sheen, confidence and presentation that new labour had. In short he's not plausible. So 50% of the voting population are left wanting some strong seismic change, without a credible politician to deliver it. Boris is no better.