I think it all comes down to density/frequency.

Believe it or not there are roads where every motorist waves at each other. They tend to be remote and passing another vehicle is somewhat rare. There's also a certain comraderie because everyone on those roads will stop to offer assitance if needed.
Pretty much the same thing goes for my cycling. In my normal commute seeing another cyclist is a rare event. I wave and give a hearty 'hello'. However, when i went downtown the other day and saw tons of cyclists... i didn't wave once.