Edit: May be this comment from Mitchell sheds some 1st-party perspective on why it may be so:
> ...Terraform is WORKFLOW agnostic, not TECHNOLOGY agnostic. This is a key part of our product philosophy that we make the 1st element of our Tao: https://www.hashicorp.com/tao-of-hashicorp
> I've talked about this more with more references in this tweet: https://twitter.com/mitchellh/status/1078682765963350016
> I don't think we've ever claimed cloud portability through "write once run anywhere;" that isn't our marketing or sales pitch and if we ever did make that claim please let me know and I'll poke some teams to correct it. Our pitch is always to just learn one workflow/tool and use it everywhere, but you explicitly WILL rewrite cloud-specific modules/code/etc.