1500 bytes is the standard MTU for Ethernet. Lots of Ethernet hardware now supports larger limits but unfortunately there is no standard for handling segments with mixed MTUs on a single vlan. So increasing the MTU from the default of 1500 bytes means that you have to make sure every device on the vlan is both capable and configured to support the larger size. That is a massive PITA.
For the most part the pain of running jumbo frames doesn't outweigh the gain. Some exceptions are networks that are used to carry tunnelled traffic and HPC/SAN networks but those kinds of specialist high performance networks don't tend to cross organisation boundaries.
There is no reason two autonomous systems can't make an arrangement to exchange packets larger than 1500 bytes but few do because it's normally not worth the cost/hassle to allow exchange of larger packets with a small number of networks (there is something of a chicken and egg situation here too).