Shared by all, and frequently occurring. The same word for what belongs to everyone and what is unremarkable.
Latin communis (shared by all, general), from com- (together) + munis (ready to serve, related to munus, duty or gift); what belongs to everyone, and by extension what is unremarkable because everyone has it.
What belongs to everyone in a shared space, and by the same root what is frequent enough to pass without comment. The two senses have never really separated.