The biggest problem is that Facebook is already a known quantity. As mentioned elsewhere, Lists and Groups can replicate most of the Circled functionality---but nobody uses them. They're not well-known, and users aren't about to start using them, even if Facebook reworks themselves: people are accustomed to sharing with everyone on Facebook.

Google+ has positioned and designed itself for Circles, and has firmly shoved Circles into everyone's skull. I think it's more of a branding thing than a technical difficulty thing at this point.