Crudely, Britain needed the support of the USA a lot more than it needed Japan'sImperial Japan isn't bombing London and in occupation of large parts of the French coast just across the English Channel from the UK and in a position to invade the UK and take out most of the British Empire's 'war-making industry' once Russia is settled. Nor is Imperial Japan actually at war with the UK up until the end of 1941, whereas Germany is.
I have to wonder if Roosevelt had some kind of blackmail material on Churchill though, the way Churchill went along with the oil embargo as he did. Maybe it was a case of Roosevelt going: 'uhhh, it would be a shame if anything happened to that lend-lease...' putting the UK in a position where the only remotely sensible position was to just declare peace all round.