That is a question for those two companies. Apple doesn't really have control over whether they develop for tvOS or not.
HBO has not made a Max Go app for any set top box. I keep hoping they'll just add Cinemax to the HBO Go/Now app and be done with it.
You can get epix on Vue also. I think it is a stand alone package, or it comes bundled with one of their offerings.
Epix, for participating cable providers, is now on Apple TV. This is a somewhat old post but I wanted to clear things up from what I have learned recently subscribing to Vue. Epix does not come with Vue, "Epix Hits" comes with Vue. Also, I know of no way to get it completely standalone. Either add to a basic plan, include with a more premium plan, or combine it with Showtime. Epix Hits, on first looks, appears to contain the same content that Epix provides to Hulu: stand-up specials, hit movies, but no original series and not every movie Epix offers.
I have no idea how it worked for me. But I do not have EPIX with my pay tv provider. But years ago I created an EPIX account with my email. I used that account to log in and it works. I have access to all the content.