Well basically all the defeated presidents since 1900 other than Ford or Trump lost by pretty large margins in the EV so really that only left Ford and Trump to make potential comebacks. The only reason Ford didnt run was cause Reagan ran otherwise he probably wins the 1980 election in a comeback imo.
Id argue HW didnt lose by much either given he lost the tipping point state by less than 5 but to most people he seemed to have lost by a a pretty easily plus he pretty much was ready to pass the torch to Jeb as soon as he lost with W as a backup if Jeb lost in 1994(which he did).
Yup, that's a good explanation. The only exception is Hoover, who attempted to win the GOP nomination at some point in 1940. But his name pretty much toxic in electoral politics post-1932, as he suffered the worst incumbent defeat in history, even way worse than Carter. If he was the nominee in 1940, FDR would have completely destroyed him again.