In that case, the spectrum was used as a replacement for the cable, in a 2 point system.
A mesh network is a completely different beast, with perhaps hundreds of thousands of nodes, spread around, and a good chuck of bandwidth being used for forwarding data between nodes.
It's not necessarily a limitation of bandwidth possible at 900MHz, it's a limitation of the normal equipment deployed and inefficiencies of mesh routing.
Those 900MHz analog phones were also usually low power, low distance, analog only devices with a few number of channels. Try having dozens of those phones all in the same room and see how useful they all are at once.
A mesh network is a completely different beast, with perhaps hundreds of thousands of nodes, spread around, and a good chuck of bandwidth being used for forwarding data between nodes.