Same reason Andrew Luck, Tim Tebow, Sam Bradford, etc. all stayed: you only get so much time in college, and he was born a Trojan fan. He probably saw what Leinart and Bush did of yesteryear and wanted to bring back the glory of those teams. The money will come. He might actually be drafted #1 now instead of #5 or wherever he is picked. While that's not a large difference, especially anymore, he'll get his money and I think genuinely loves USC. It's not some conspiracy theory where Kiffin paid Barkley to stay his final year and give them a shot at the national title. He's just following the footsteps of some lately and wants to spend more time in college. No problem with that. If he doesn't feel ready for the NFL he didn't have to go, so he didn't.
Oh, to be more direct with your question, USC's sanctions are up starting next year.
I'll also address the USC/Barkley issue since I know WAY more about it than anyone on this thread. (not bragging, I just cover these guys for a living).
1. Barkley was ready to go pro. In fact, Barkley was GOING pro. It wasn't until the 11th hour that Barkley had a change of heart and decided that he would listen to everyone that told him to follow his heart. The decision was made to enter the NFL draft and Barkley intended on doing that, but he had a sudden change of heart and stayed for his Senior season.
2. The biggest advice that Barkley received in this process was for him to listen to his heart. Matt is a highly religious person and felt that it was God's plan for him to stay since he began to feel a change of heart. He decided he would "listen to God" and informed Kiffin late at night that he would be returning. I am not advocating religion or spouting off what *I* believe happened, I am telling you what he's gone on record saying and has been reported by USC personnel.
3.
"Unfinished Business": The Trojans will return every skill player with the exception of Matt Kalil and Nick Perry. Everyone knew that Perry was gone, but a lot of people felt like Kalil would return if Barkley returned and vice versa. Kalil made the smart move. He's the best offensive lineman in the draft and he's a great special teams player (blocked at least 4 kicks). With Robert Woods, Marqise Lee, and Curtis McNeal (USC's leading rusher) all coming back and losing only Kalil off an offensive line that gave up the fewest sacks in the country, Barkley knew the weapons were in place. Not to mention that 5 star recruit George Farmer will have an entire camp under his belt and shifted to the wide receiver position full-time headed into spring, it's an embarrassment of riches on offense.
You couple that with T.J. McDonald returning to the safety position, the signing of JuCo standout Gerald Bowman (nation's top safety & defensive player at the JuCo level), plus only losing Nick Perry off the defensive line and you have another embarrassment of riches on defense.
USC has added Morgan Breslin, a stud defensive end out of Diablo Valley C.C., and has 6'5", 340lb tackle Jordan Simmons out of Crespi signing with them too. With the 6'6", 375lb Walker expected to move from Right to Left tackle, there won't be much of a drop off protecting Barkley's blind side headed into 2012.
USC's Freshman kicker, Andre Heidari, was good enough to make the Pac-12 first team and has been kicking 60-70 yard field goals in the offseason. (They were unabated and nobody was rushing him, but the distance was there and that's not something you see from a 19 year old kicker). I have video if people doubt me.
USC will be the nation's best team in terms of what's returning, what has already produced that is there, and what's incoming (despite scholarship sanctions). Whether or not they can piece that together on the field next season and make a legitimate run at the title will remain to be seen.
The balance of power has shifted to the North, but you can expect to see a significant drop-off at Stanford. Oregon will lose several play-makers and suffer. Look for Washington to surge with run stoppers signed at the JuCo level and for Coach Leach to get him some offensive stand-outs up at Wazzou.
Outside of USC, only two programs will be returning their coach next season in the Pac-12 South. One of them is Colorado. Enough said.
THAT is why Barkley decided to stay.