What I "like that much" is the pragmatism (which, again, I didn't write) of the choice to break the JSON standard and introduce features that are significant for configuration files (comments, essentially, and to a very minor extend, trailing newlines in arrays).

I've never seen Deco, but it's not an established standard. Based on the repository, it has no grammar and it even leaves details to the implementations (e.g. multi-line strings). The simplicity comes at a cost, for example, to represent leading whitespaces.

If one asks 10 developers which format they'd use, they'd choose 10 different ones, therefore a certain level of standardization is required.