Evil Sibling
And sometimes it's a matter of character.
Yes could have set up an in-house streaming brand and market it as an additional low-cost service, alongside its satellite and content broadcasts. Were it so, it is pretty likely that its character would have taken over the brand, framing is as not as important or lucrative.
Also, when interacting with its target audience, Yes would still be perceived as an 'established' service, and customers who wish to disconnect from “the establishment” would probably turn to others.
By launching Sting separately and allowing it to grow as a completely independent entity, Yes actually enabled Sting to rebel against them and appeal to an audience with different desires and feelings. Separation, in this case, enabled innovation in approach, thought, and brand, which proved crucial to this service and product.