Well besides the already mentioned of higher paydays and health...there are a bunch of other factors....Most take place once a fighter has hit the top 10 or near the top 10...

Strategy--Fighting the right opponent will get you a shot quicker if you win...if you lose to that guy it does not set you back too far....Sometimes you have to wait for the opp to fight them...

Titles--with so many titles around the wrong fight even if it is a win can hurt your rankings...

Politics--everyone is so picky and choosey now a days...this guy wants the venue..that guy wants glove choice...if both are given what they want something else comes up...then no one wants to budge on what to give in to...causing delays...

Add the hold ups by state license authorities, medical exams, or what ever else you could possibly think of

Again though it all leads back to money...Carefully thought out careers will earn the big bucks...fighting just for the sake of fighting will earn you money but not the right money....

Truth be told the fight itself is the easiest part of the whole process...