At least with Cameron he was on the right side of the argument. He wrongly assumed the big hitter's in the party would help campaign for EU membership and the issue could be put to bed.
What he failed to account for was Russian dirty money and influence. And other tories who didn't really believe in Brexit jumping on the bandwagon as they new it was the best bet fir a future stab at the top job.
Johnson wanted remain to win but in campaigning to leave he wanted to win over the tory base. That self interest is what is now killing the tory party. Johnson had no idea what to do with Brexit.