MMA is always gonna be structured around the spectacles. It can't have a regular schedule like normal sports because "games" can't be played regularly for health reasons. And the biggest spectacles are the ones containing the biggest stars. Because of that, the UFC is only as big as its biggest stars at the time. Basically, 2014 had no stars and 2015 had Conor and Ronda.
In 2014, both Silva and GSP were absent. Jones only fought once. Conor and Ronda were only rising as stars and not yet PPV draws. There were also injury problems and a record high amount of shows, spreading the cards thin. On top of that, there was neither a new year show at the start or end of the year, as UFC 168 fell in 2013 and UFC 182 fell in 2015.
In 2015, Ronda Rousey became the biggest mainstream star in UFC history, and a PPV draw, bringing in nearly 3 million in PPV buys, and setting a new attendance record. Conor McGregor became the biggest star in certain ways, bringing in huge PPV numbers himself as well as record-setting gate numbers. UFC 189, UFC 193, and UFC 194 are among the most historically significant shows ever, and they all happened in 2015. Jones/DC and Anderson Silva's return against Diaz also brought the PPV numbers. 2015 could be a tough year to top, but we'll see if the UFC can do it this year with UFC 200, multiple Conor fights, potentially multiple Jones and Silva fights, Rousey/Holm II, and maybe the return of GSP.