The chromecast isn't necessarily something that brings revenue on its own -- it just lowers the barrier to entry for Google's other services. If it's easier for you to watch youtube, play Google Music, etc. on your TV, it's more likely you will do so... and be exposed to ads in the process. I'd liken it to Google Fiber.
It probably leverages Android in some fashion also, though good iOS apps have chrome cast support as well.