Sanctioning rules state you can only fight every two weeks. Not that anyone fights that often. I think a lot of fighters realize the toll it can take on your body thus they fight less.
Unavailability creates demand. They wouldn't make as much if they were in a bunch of keep-busy fights.
Health concerns are a big reason fighters don't regularly fight as much as they used to. With all the study and research that has been done on the effects of wear and tear on fighters, especially the neurological impact of having someone's fist rammed into your skull for a living, can be devastating. Great care is taken to make sure these athletes can have a life outside the ring after they are done boxing (although there are exceptions).
They are getting paid well enough at the top level that they don't need to put their body through the abuse of two hundred fights. At lower levels they fight more frequently.
Money you don't need now to fight so often because you gain more money per fight than in older years, and i thin a fighter after they fought they get a mandatory suspension because they need a week or two to fight again, but i don't know too mucho about that.
BIG fights takes too long because of Promotions and Negotiations. So any fighter who gets in big fights could only fight 2-3 times a year(cept some few boxers who fight 1 time or none). Normals boxers could fight 3-5 times a year.
infrequency and the reaction to a loss is what is killing boxing. look at MMA chuck liddell lost to quinton jackson, then keith jardine about 4 months later, then went on to fight wanderli silva and won it all in one year. he lost again but the point is the fans have been behind him the whole way and are still willing to buy his fights. now tell me if floyd mayweather didnt have that "0" with him would he be as big of a name? maybe not so i think the fear of a loss is one of the reasons why boxers fight infrequently. and one of the things that is killing the sport.
As everyone else has said, it comes down to money. At the very top of the game it's pretty much impossible to fight 5 times a year. Even if the fighter wanted to, his manager or promoter probably wouldn't let him. Boxers are out to make money for themselves first and foremost. That's more likely to happen fighting twice a year fighting on ppv.