I don't think XML was all that widely used, and the implementation looked more "looks interesting, let's develop something and people will use that".

With JSONB it's more like "people are already using JSON data type (text + validations) a lot, and want something better, and we do have hstore, and we're working on hstore 2.0, so let's use the ideas for json". And that's how JSONB was born.

My estimate is there are already more people using JSONB than XML (talking about the PostgreSQL data types), so the momentum is very different.