Tyson had a couple of times where he fought twice a month.
It's not uncommon for new pro's to fight once a month. Floyd fought just about once a month until he fought Genero.
But the business has changed. Once you become a viable and marketable contender, then promoters/management tend to scale back the frequency of bouts. Partly to protect the fighter, but also to allow for time to promote each individual fight and build interest.
If Manny fought every month, his events wouldn't be as big as they are (even though his numbers have declined, he's still one of the biggest draws in the sport). So they are building more interest, given more time to promote, and also reducing the chances for a loss.