I can't speak for everyone, but for some it's a case of never being able to get your brain to shut down properly from a work point of view, which can be both the reason for sleep problems and the reason for success.

My current boss summed it up once when he said "the reason I'm always working outside hours [he works most evenings, will often get an email from him at 3am, most days a call sometime between 8pm and midnight - and this isn't some young startup hacker, he's a successful CEO of 10 years] is that I'm afraid of missing any opportunities".

Another way of putting that is Carpe Diem, one of my favourite quotes.

I'm the same, no matter what I'm doing - including trying to get to sleep - thoughts keep cropping up. Should we be talking to Company X. Do we need to hire Person Y. Why haven't we created Product Z.