When he came back to the US from France in 1785, he was very much hoping to retire from public duties. Instead, Pennsylvania made him President of the state, ( essentially the governor) and he served three terms, for three years, and complained of the work. Even if he had been willing, once he had seen the Constitutional Convention finish in 1787 his health was too poor to be considered for first President. But there was no one running against Washington, anyway: the only competition was between candidates for Vice President

As alluded to in the other comment, it's more that it could've been no on other than Washington.

Also, remember that the way we see and revere someone in the present isn't the same that they were seen at the time. Franklin was well-regarded, but no one had a national profile like Washington.

Plus, his age. He was just over 80 in the late 1780s.

Ben Franklin was 26 years older than George Washington - old enough to be his father - and if Franklin was ever going to be President, he would have had to have been the first. But he did not stand for election, and at any rate, George Washington received 100% of the votes from the Electoral College.