The main alternative was and still is Cloud Foundry (by way of disclosure, I've been in and around CF for years). But it's always been pitched to enterprise customers and approximately zero effort has ever been made to expand awareness outside that world. Kubernetes had Google's massively powerful halo blowing in its sails.

> ...why is Heroku/buildpack not running away with it?

I predict that buildpacks are going to make a big comeback, because they make everything about Day 2 much easier. Pivotal and Heroku have been cooperating on the Cloud Native Buildpacks[0] spec under the CNCF sandbox process. There is some seriously cool stuff coming down the pipe.

[0] https://buildpacks.io/

Personally, I stopped using Heroku because it is more expensive than manually deploying to something like Digital Ocean. I also found dealing with third-party services that were here today and gone tomorrow a little irritating. Maybe things have improved recently. I haven't used Heroku for a few years.

There's no question that you can build higher-level deployment strategies on top of the system (or any other system). It's just not baked in.

Also, YAML in general is not a bad thing, but kubernetes presents you with a ton of boilerplate and a complicated DSL. There's a lot of rough edges (annotations don't get validated, for example).