Sometimes people just only want those that follow them to view their profile, or no one to view their profile. Be it because they don't want someone to post a message on their profile, or maybe they like the 'security' of it so they aren't being stalked.
There could be many reasons as to why someone may hide their profile from the general public though. Some people go so far on stalking as to go to a profile, hit the little 'recent posts' tab to try and track when they are online or if they are online.