Why is this a language, not just a framework for JavaScript or whatever?

I suppose you could say the same about Terraform using HCL - but that makes extensive use of it being declarative, which is so unusual that they'd be picking an obscure probably-new-to-the-user language anyway. This just looks.. JS-ish, a bit like anything really.

I suppose I'm asking what are the language features that make Wing great for 'cloud', any `new Bucket()` etc. aside?

If you look at its 'design tenets', the only ones that are about the language not the SDK are just saying 'be good/familiar like other languages'... Ok so why not be another language? https://docs.winglang.io/reference/spec#02-design-tenets