Because VP9 is much, much cheaper for:

- Google, who doesn't have to pay royalties

- Google and consumers, who can enjoy better compression and lower bandwidth

- Consumers, who can enjoy a much more mainstream video encoding format in not just YT but pretty much every app.

Google doesn't want to write off 45% of the set-top market right away, but at the same time it's 100% in the right to demand Roku support modern royalty-free codecs going forward.

Roku fights pretty much everybody nowadays and as someone who's been dealing with full-screen ads and missing apps on my $1000 TV, I have no sympathy for Roku whining about needing to support a modern codec.