Brock lesnar was hardly a big star. Just because WWE handed him everything doesn't make him a big anything. The only people that think he was ever a big deal are the pea-brained idiots that WWE is catering to right now.
It's real simple. brock lesnar sucks. he never deserved the championship because he never earned the championship. 4 months with the company and then he gets booked to go over WWE's top guys is what I call B-U-L-L-F-R-E-A-K-I-N-G-C-R-A-P and L-A-Z-Y-A-S-S B-O-O-K-I-N-G AT IT'S WORST! That is not the way to make a world champ and nobody can tell me different!!!
Is your name Wade Keller by any chance?
ity1:
Brock was a big star for the two years that he was around. He would have been bigger if he had stuck around for longer since it would meant more high profile feuds with the likes of Triple H, Shawn Michaels, John Cena, Undertaker (a streak match at some point would have seemed imminent), Batista, etc. Also, if he didn't leave after Wrestlemania XX, then his match with Goldberg would have probably been a lot more well-received by the MSG crowd than it was (given that the crowd shit all over it because they knew both guys were taking off afterwards) and he would have likely put in more effort into making it worthwhile.
Brock was pushed as fast he was because he was the perfect prototype of a WWE Superstar, plus (as mentioned) they needed some fresh blood in the main event at that time. And it's hard to picture a guy the size of Lesnar floundering about in the midcard for too long before people naturally start wondering why a guy like this isn't already main eventing.