Much of the French interest in persuing European integration had been to boost its economy and to keep itself surrounded in Europe with friendly powers, so that France could project power abroad and remain, if not a superpower, then at least a nation of sufficient weight that it must be respected. Leaving the EU means possibly putting a bullet in the head of decades of entanglement between its economy and the economies of its neighbours, as well as taking itself off of the scales that are currently balanced between the pro-Russia and anti-Russia factions of Europe. If France is stuck trying to work out how its economy and international trade is supposed to function, and within the EU it's Germany, Britain, and Italy pushing for greater collaboration with Russia with the only voices of resistance being a bunch of poor post-Communist newbies, then it is really difficult to see how France's geopolitical position is improved over where it is now, where already Russia has displayed the leverage to crowbar France's neo-empire in Africa away from it. And yes, Brexit was also a clearly idiotic move that still happened anyway, but it should be clear that the situation is not so comparable to Britain IOTL. Our EU at least is fairly united on its position regarding Russia, the only main difference is how intense that suspicion and disapproval is, France would have to know where the consensus in the EU will shift without them supplying the No. 2 voice. Also, France was occupied in the Second World War, it fully knows, culturally as well as politically, how badly things can go for itself if it cocks up its geopolitical strategy, a French President in David Cameron's position wouldn't think to run France through such risks, probably even if they knew it would cost them the election. Better to have won and then crushingly lose after one term, than to earn two terms and use them to force France to give up much of the international influence it has.