Despite Lives of the Twelve Caesars there were far more emperors than months to name after them.
it is not certain to me that Gaius Julius Caesar and Augustus ordered months named after themselves. It is possible that those honors ere decreed posthumously by the senate (allegedly without being prodded by the heir) - you should look it up. The senate was only likely to name a month after an emperor they liked, usually when posthumously decreeing him a god. If an emperor just arbitrarily named a month after himself without going though the senate the next reign might have that action reversed to gain favor with the senate.
A lot of unpopular emperors were posthumously punished with Damnato Memoria when their decrees were nullified and all inscriptions and monuments honoring them were erased and destroyed. If that was done, nothing, especially part of the calendar, would be left named after that emperor.
Curiously, the ins and out of imperial politics meant that the list of Emperors decreed to be gods, likely to have months named after them, and the list of emperors punished with Damnato Memoria and having everything named after them renamed, largely overlapped and did include some of the same emperors.
And the renaming of months seems more like a custom of the early empire and Commodus was probably the last emperor to rename a month after himself.
And after the Emperors became Christians in the 4th century AD they stopped being decreed gods after death and that probably made it very unlikely that months would be renamed after them.