I'm just not comfortable making a format basically nobody knows, which was introduced less than a month ago as "an additive change", which isn't as well documented and indexed as the json one is (http://code.dlang.org/package-format) the default.
In addition the likeliness that few bugs will arise doesn't mean that none will, and I'd take a used-over-the-years solution over something just implemented any day.
`dub init` is likely to be the first thing a beginner will want to do as soon as (s)he learns about code.dlang.org, and event if it's still possible to use json, it's not well documented / known yet, so I don't see a clear return on investment on this change, while I can clearly see a net loss.

Obviously I'm not saying the format is not suitable, but I'd like to give it more time to mature.