They aren't really actively marketing it, and Android TV is a fundamentally different product. A Chromecast is a dumb wireless adapter for your TV with a minimal interface largely controlled by a phone, while an Android TV (and Fire TV, Apple TV, etc.) is a small computer designed around a 10-foot interface.

The Roku is in a unique spot: it's got a channel store, so it can be expanded easily, and it can be operated with only a TV (no phone or tablet required), but isn't to the point where it can run full apps. It'll be interesting to see what happens, given the Fire TV (excluding the stick, which is closer to a Roku) and current iterations of the Android TV have been sales disappointments.