ABC is a Television Network and Direct TV is a Television Provider. TV networks sell it's programing to TV Providers. Most TV Networks have two sales tiers, Device and Streaming. Direct TV likely only purchased the Device tier. That is why you can watch it in other places but not on Roku ABC app.
Also, ABC/Disney is always getting removed from Cable and Satellite systems because the two entities can never agree on a price. I'm assuming Direct TV did not want to pay the price for streaming.
Make sure you provide feedback about this to Direct TV or it will never get resolved.
