why should they pick a javascript engine?

The article -- in fact, almost the entire point of the article -- describes why the situation is not that simple... These engines were designed to be used within the context of a web browser (embedded, single-purpose), so as Javascript becomes more relevant outside the web context, the language and runtimes will grow additional features -- thus the features provided by Mozilla's implementation which are attractive to the GNOME Shell hackers.

Here's an illustrative challenge for you: How do you write to stdout in Javascript, and if you can answer that, is your method a feature of the langauge or the runtime?