There are actually multiple implementation of networking over ham radio (though not using teletext).

Some of the limitations are that ham radio requires getting a license (it's easy, but it's a little bit of work and turns some people off), the user base is tiny (it's a niche inside a niche), it requires technical knowledge and specialized hardware, and legally it can't be encrypted or used for commercial purposes. That's okay if your plan is to broadcast messages without censorship, but not so great if you want to check email or browse https sites.

https://spectrum.ieee.org/build-a-longdistance-data-network-...

https://www.reddit.com/r/amateurradio/comments/5bj5w0/intern...