Comment by yllus

To draw anecdotally from my own experiences, its for two reasons:

1. It's simple to get started with, but complex enough to tweak to your needs in respect to simplicity of deployment, scaling and resource definition.

2. It's appealingly cloud-agnostic just at the time where multiple cloud providers are all becoming viable and competitive.

I think it's more #2 and #1; as always, timing is everything.