The incentives were the simple (and common) political expediency of using the EU and immigration as scapegoats; whether to excuse their own failings in power or attack the Blair government. Many politicians did that for years without seeing any benefit at all to actually leaving the EU, but as David Cameron found, if you spend long enough blaming the EU for stuff, sooner or later you're going to get cornered into "why don't you do something about it?". And public concern about 'mass immigration' is less about incentive structures to change it and more a constant that seems to exist irrespective of where, when and what the actual levels of immigration are, which results in massive boosts to the political profile of anyone who wishes to talk about it.
There are people with obvious financial incentives to get a trade deal with the US (not necessarily on terms beneficial to the UK), and people with obvious political incentives to destabilise Western European liberal democracy, but they were largely in the background. And some prominent Leave campaigners made a lot of money shorting the pound because they believed they would win, but that was just as much a gamble on the result as anyone else directly or indirectly betting on it.
