900 MHZ ? I guess nobody uses it. (?) I'd guess not enough amateurs to cover all bands. (?)
It pretty simple. Out of all the Hams I have talked to most don't want to bother with anything that doesn't involve chucking a wire in a tree and talking (wait for it............ here it comes) around the world. There are what? almost 3/4 of a million license holders? (I read somewhere). There are plenty of Hams to occupy those bands. Unfortunately, I know of Hams who are in the process of upgrading their license or have upgraded their license because they literally think that the only privileges they have are a tiny portion on 10m, 6m, 2m, and 70cm. They literally have no idea that they have almost 2 GHz worth of bands to play with. Technicians have all privileges and all modes on all Ham bands from 50 MHz on up.
The issue is that most get their ticket, jump on 2 meters, try to have a conversation and find that no one is there except for one or two guys who tell them they have to upgrade to go where the action is - on HF. Once they do that, they almost never bother with anything above 10 meters. Now multiply that by how many Hams do that and well....... you want to know why there are no Hams using these very local shorter wavelength bands....... That HF contesting..... Its like a drug no one can kick.
Personally I would love to experiment with a lot of those bands 1.25m, 23cm, and higher. However it takes two people to talk, and even if it is a one person activity such as the mesh networks you can work with, I am about as familiar with it as a 100 year old person is with building an operating system for a computer from scratch. I would need a lot of help. When everyone is on HF and they refuse to play with anything up there and you are in the bands that literally don't get used with the exception of a few groups spread out over the whole United States, well, your Ham Radio experience will be not so good. This is why the FCC takes Ham Bands away. Some rich billionaire or company comes along needing frequencies and are willing to pay. Hams aren't using the bands? Well, you know where those frequencies go. Money talks and you know what walks. Its the sad truth. I feel real bad for those who actually do use those bands or want to use them. One day we wont have any of them. Already, 900 MHz is a shared band with low power devices and intercoms last I knew.