Although they won't admit it, the reason was obviously having more control of North sea oil (and fish).
This is why a lot of remainers thought we were simply going to be in the EEA like Norway and Iceland, but there's more to it than that, like expanding trade with North America and Australasia. Not so much a return to the British empire, but stronger connections with those countries.
Don't get me wrong, I love Europe, but I hate the EU. Their plan was a pipeline from Russia to Europe for cheap oil because Britain kept saying "you're not taking our oil" and the final nail was when Putin flat old told the EU, "you're not taking our oil."
There is of course the immigration crisis and the Irish border, but can we at least discuss the elephant in the room that this whole debacle was about oil?