The broadcasters have not permitted it. That is the primary reason why casting isn't available at present.
I want to add to this thought. I feel Telus should push more on this. By logging in and using the Chrome browser, it casts successfully anyway. So it doesn't feel like a great answer to the problem.
What is missing on the app side, which should be overcome able and benefits the broadcaster as much as Telus.
The brodcaster controls how they want their content viewed. It has nothing to do with Telus. The content provider has the last word. If they say no casting it’s no casting.
I’m not a fan of casting. I prefer UI of an android tv device for my content.