It’s simple: not all websites are protected very well.
“You’re only as strong as your weakest link.”
Say you use any or all of these services: Dropbox, Facebook, iCloud, Google Mail, Twitter and some less popular websites and you have the same account information for all of these. Chances are a hacker will try to hack his way into the less popular websites and extract e-mail addresses and passwords here. Imagine, he now has access to your Dropbox files, Facebook account, e-mail and all the others.
So, do not use the same account across the web.
Granted, it’s becoming harder and harder as standards are set higher, but it’s still very realistic.