I wouldn't say it's the only reason. H264/5 are not royalty-free codecs. YouTube is a powerful tool Google can use to push a royalty-free codec, much like Apple used the iPhone to push people off Flash. It's one of few areas I agree with Google being heavy handed.

What, over and above the H.264 licensing that covers them for 1080p content?

It’s google doing what google always does: pushing a google controlled thing to become a “standard”.

It's an open source, royalty free codec. I'd much rather use that than one owned and licensed by a private organisation, no matter whether Google develops it or not.

It’s still patent encumbered, and if you happen to sue Google, because they do something shitty, you lose your patent grant.

How does that make H264 preferable?

It's not operated by a company with a penchant for abusing its 800 pound gorilla status?

Apple's MacBooks also got VP9 hardware decode support since several years, Apple could just enable it on the software side.

But they don't, so Safari users have to suffer.

Is it unreasonable of Google not to want to use a royalty free codec that's controlled by the MPAA?

They could, but they don’t, so what is your point? It still means that watching YouTube on the Mac device in question is a crappy experience.