There' a few reasons:

1) Channels like CBC Newsworld receive payment based on the number of cable satellite subscribers they reach. They wouldn't want to give it away for free. They want you to subscribe so they get money from you.

2) All specialty channels are not licensed for OTA distribution even if they wanted to.

3) I don't think any broadcaster has even though about how they could make money off of offering a subchannel. Most do OTA because they have to, not because they want to. I wouldn't be surprised if even the engineering staff at OTA stations themselves subscribe to cable.