I wouldn't take that as a reflection of rudeness. That simply means that you might be on the wrong side of the "Grüezigrenze" (this is a made up word I use which describes an imaginary border which determines whether strangers will greet you in public).

You see where I live I'm not in the city itself, but I'm close to it. I seem to be on the wrong side of the grüezigrenze so when I occassionaly forget this and greet people I'm sometimes ignored. This is a similiar dynamic that plays out all over the world - try greeting a Londoner or New Yorker and see what happens.

But rest assured, when walking in less populated areas you'd better get your grüezi ready and make sure it is well polished - you'll need it.