6 meters seems to be a place where when the conditions are right, repeaters may interfere with each other from far distances, but when the conditions are normal, repeaters are similar to 2 meters.
What is the advantage of installing repeaters on 6 meters?
6m works better than 2m when dealing with terrain. For direct line of site, any of the VHF or UHF repeaters will work essentially equally well. Their significant difference comes when one is dealing with obstacles. The larger the wavelength, the more ground effect there will be, which essentially allows the radio wave to hug the ground and thus be propagated further. 6m repeaters are thus more popular in mountainous areas.
Incidentally, UHF does a better job of going through a building. The reason is, the signal can find smaller holes than the VHF can (To oversimplify it). There are a lot of little quirks in the different bands, which is why there are so many bands available to Amateurs, to learn all of these quirks and maximize communication possibilities.
