Not trying to be snipey here, but it's because there is no generally agreed upon channel. Don't get me wrong, I'm not against 20. I'm hoping it becomes the undisputed calling king of what turns out to be very busy GMRS airways. I'm just saying, by virtue of the fact so many people don't "know" and we are discussing it, don't you think it's still a bit of an unknown? Saying "I declare it so" or seeing it on some other forum from some guy who says so doesn't make it so.
I was joking about 19, but I now officially support 20 for a "road" channel. I hope it succeeds.