Brock had nowhere near the consistency that GSP had.
GSP also sold more PPVs overall.
Obviously GSP sold more PPVs overall. He headlined three times the number of events that Brock did. The question was who was more popular, not who had the most success (therefore leading to more PPV headlining slots) in the UFC.
So, just to show you how wrong you are, here are the numbers. Naturally, I've excluded UFC 100 since both appeared on that card. And if you want to say that GSP fought in earlier years when MMA wasn't as popular or some shit, I'll only include his run from 2008-2011 when Brock was fighting
2010, Brock/Carwin: 1.16 million
2010, Brock/Cain: 1.05 million
2008, Brock/Couture: 1.01 million
2011, Brock/Overeem: 750k (this also wasn't a title fight)
Average Buys: 992.5k
2009, GSP/Penn 2: 920k
2011, GSP/Shields: 800k
2010, GSP/Koscheck 2: 785k
2010, GSP/Hardy: 770k
2008, GSP/Fitch: 625k
2008, GSP/Serra 2: 530k
Average Buys 738.33k
And tf do you mean by "nowhere near the consistency"? Brock did consistently better than GSP, who didn't headline a single PPV that eclipsed Brock's average buy rate. If he didn't pull consistently high numbers, they wouldn't have put him in a PPV main event slot for a non title fight after he had lost the belt against a fighter making his UFC debut.