Why didn’t Unitarianism become mainstream?

I wonder about this. Unitarianism gained significant popularity, especially in the late 1700s. Four Presidents of the United States were Unitarians: John Adams, John Quincy Adams, Millard Fillmore, and William Howard Taft. Adlai Stevenson II, the Democratic presidential candidate in 1952 and 1956, was also a publicly known Unitarian. This clearly shows that Unitarianism was not viewed the same way by the American public as it is now. After all, it wouldn’t be politically rational for a head of state to belong to a heterodox denomination.

Why did Unitarianism lose all of its congregations when it was once widespread and held serious prestige? Today, no one is left in Unitarian churches, and those who remain have almost entirely turned into Queer churches.