To really sum it up it's 60% business/entertainment and 40% sport. There isn't a ruling body for official rankings nor a true ladder system. Factor in injuries, mother nature (Akiyama couldn't fight because of what happened in his homeland + Weidman losing his house in the hurricane, etc), suspensions, extraneous circumstances and you get what you have currently. A lot of fights get changed around.
It's best not to think about it. Imagine Joe Silva and Dana White what they have to deal with on a daily basis. They're constantly shuffling the line-ups out of necessity.
GSP vs Condit pulled in 700,000 ppvs
GSP vs Hendricks = 500,000 PPVS (The numbers will be reduced fairly significantly. Everyone buys PPVs to watch GSP fight, but it helps with a background story these days.)
GSP vs Diaz = 800,000 (My own estimation.)
$50 x 300,000 = $15,000,000
OP. My figures shouldn't be too far off. Factor in ticket sales as well which should be sold out. What would you do as a fighter, manager, trainer, and fight promoter.
Dana asked GSP what he wanted to do as a sign of respect. If he chose Hendricks I have no doubt it would have been greenlighted. GSP wants to pick the easier route this time around, but you know what he'll face Diaz sooner or later.
It's the path with least resistance. Sadly fighters like Hendricks and Weidman will have to continue marching forward. They NEED to understand that the current champions followed the same path for over a decade and they EARNED their stripes through battle. They gotta wait in line although they're both very close to title shots.
If you can't see the big picture then I dunno what else to say.
GSP/Hendricks would have done more than 500,000. Probably 700,000 which has become the usual for GSP fights. Hendricks is a pretty easy guy to build up too and is a much more interesting fight than Diaz.