It can be for many reason:

1 silly mistake - Everyone has an off day or makes silly mistake
Other hackers grass them up - People get jealous or caught and make deals with FBI,Police so on

Someone better - There is a lot to know about security no one knows everything

I am sure there lots of other reason 

but there are always smarter people as long you have money to employee them.

Lets say you member of anonymous and you use a VPN service thinking you all 1337 the FBI just go to that provider and say we want these records and get your details.

Look that guy who got caught by FBI he posted a picture of his GF online and they grabed the image and got the geo tagging information from the picture that gave them the exact GPS position of where the picture was taken. that how he got busted

Then you got the other guy not sure how he got busted but he mad a deal with FBI and help them catch other members.

yes but people always drop their guard it just human nature. Some of it as well is when people don't get caught they get greedy and think they never get caught so take more risk its same reason why most bank robbers get caught and other criminals they always take that one step too many.

hmm I guess but not everyone who get caughty is not just script kiddie. Kevin micknick got caughty I would never class him as script kiddie. I do agree that most probabley are the fact is everyone makes mistakes and that is usally their downdfall.

As already mentioned before carelessness or greed. Sabu of LulzSec got caught because he logged into IRC before going through Tor.

http://www.americablog.com/2012/03/how-fbi-caught-lulzsec-hacker-sabu-and.html

"hmm I guess but not everyone who get caughty is not just script kiddie. "

I agree! But a lot of the ones that get done are just script kiddies and thats why they get caught. They run tools and sploits without knowing what they do and the mess they make in logs!

People like Kevin are not in the same class as a lot of the guys who get done today..... He is a true talent!