They have trouble ("lag") due to distance---if connecting to another country, which obviously means anyone other than AU.
Distance itself is a factor in ping, but not the only one certainly.
Ping would be good because it isn't just dependent on geographical distance, but also the line condition and distance of the route itself. If there are other factors influencing the connection, ping can reflect that. I don't want to connect to someone nearby just because they're geographically closer if there are line conditions resulting in a slower ping reply.
If I see a ping of 120ms, I don't want to connect to them regardless of where they are. However, if I get 55ms, I couldn't care less. Realistically, geography will come into play though, so it will all take care of itself.