Normally when the UFC has two title fights on deck for a PPV event, the bigger weight class gets the top billing over the smaller one. The only recent occurrence of the larger weight class getting co-main event status was UFC 111 in 2010, when an interim title fight between Frank Mir and Shane Carwin was placed behind the main event of Georges St-Pierre and Dan Hardy. Otherwise, UFC 100, 136, 152, etc. have generally adhered to this standard. That will not be the case for UFC 169 on February 1st, as the UFC has confirmed Dominick Cruz/Renan Barao will be the main event ahead of Jose Aldo/Ricardo Lamas:
#UFC169 main event will be the champion vs. champion bout: Cruz vs. Barao. Aldo vs. Lamas will be co-main.— UFC News (@UFCNews) November 5, 2013
There could be two very legitimate reasons for this move:
1.) Cruz vs. Barao is a "title unification bout", thus has a bigger storyline. Dominick Cruz hasn't fought since the UFC was on Versus (October 2011), and the UFC chose not to strip him of his belt. Renan Barao took apart Brad Pickett, Scott Jorgensen, Urijah Faber, Michael McDonald, and Eddie Wineland during Cruz's inactivity, and has defended his interim belt twice. We just had a title unification fight between GSP and Carlos Condit, and this would fit a similar mold (but at a smaller scale both in weight class and in name value).
2.) Jose Aldo's PPV numbers are terrible. Keep in mind that Cruz has only one PPV main event (vs. Urijah Faber at UFC 132), so it's not exactly conclusive as to what Cruz can consistently pull in. However, we know what Aldo offers as a PPV commodity, and it's not been pretty. These are the reported buy-rates for all of Aldo's main event fights:
This doesn't even include his co-main event fight with Kenny Florian at UFC 136, which did only 225,000 buys despite two title fights and Chael Sonnen on the PPV. His only measure of success was his UFC debut against Mark Hominick at UFC 129 in Toronto, where he was co-main to GSP/Jake Shields for a PPV that would ultimately hit 900,000 buys. UFC 132: Cruz vs. Faber did a reported 350,000 buys, which is higher than all of Aldo's main events, and when you look at UFC 132 compared to UFC 156, the latter was a significantly deeper card with more "name" fighters. The odds of Jose Aldo's numbers improving against Ricardo Lamas, who until his last fight had never been on a UFC main card (of any sort), are slim and none.
In the grand scheme of things, this is not really a big deal, but it is worth noting what the UFC values as a more "sellable" main event, and it's not the current top 3 pound-for-pound ranked fighter in the world.