Comparing any of these trails to the AT reveals a lot of what's wonderful and not-so-wonderful about the AT.

They don't have the crowds or the traffic. They often don't have shelters. They often have lots of roadwalks. Compared to the AT, they're often poorly maintained, poorly marked, and subject to frequent re-routes. They travel extensively over private land.

Because of this, some of them aren't really amenable to thru-hiking. They're meant more for day use.

They simply don't have the massive volunteer force that ATC and its many member clubs can bring to bear. Historically, the AT is an amazing, wonderful and unique thing.