We are in an era where the HW division is weaker than the HW divisions from the previous two generations spanning back to 2004. Derrick Lewis and Tuivasa are top 3 HWs today. They wouldn't be top 10 in 2013 and they woudlnt be top 10 in 2005.
The LHW division is also weaker today than the last two generations. Today's version of Glover Texeira is the champion at 43 years old and took the belt from a 39 year old journeyman Jan. Not to mention journeyman MWs like Anthony Smith still hanging around the top 5 at LHW. Today's Glover and Jan wouldnt be champions from 2011-2015 and they wouldn't have been champs from 2005 -2010. Anthony Smith wouldn't even be in the top 5 in the last two generations.
The MW division today is weaker than the MW division from the previous generation from 2010-2014. Guys like Vettori wouldnt be title contenders from 2010-2014 when you had Anderson, Hendo, Vitor, Jacare, Weidman, Romero, Rockhold and Bisping.
I don't know why but the talent level is lower today across the board. The smaller divisions are a lot better today. LW is better than ever. BW is better than ever but these guys also make a lot less money than the big guys. The bigger athletes are obviously seeing more opportunities in other sports.