Just as the title suggests. The source editor on mobile doesn't have many helpful features. None to be precise. It only lets the user edit the source and save it. Or switch to the visual editor. However 2017 wikitext editor comes with several features, it is very helpful for inserting wiki markup in a page easily. True, mobile visual editor has similar functionalities, but mobile source editor doesn't have any kind of function whatsoever.
We were planning this at some point, but other features took priority.
At the time we felt that maintaining both the old and the new wikitext editor on mobile would be a big effort, and we didn't want to replace the old with the new one because we weren't sure if it was an improvement, particularly for users with slow devices or slow internet connections.
I'm not sure if folks still feel the same way today :)