I'm not sure, but a concern has been expressed in the past that certain development efforts had been focused on Wikipedia related functionality, as opposed to "other projects". I am not aware how many active developers there are for Wikisource related things, but I (not a developer) had filed a number of Phab tickets that got (not always favourable) responses, related to issues I'd run into with Wikisource related use cases.
A few of the issues at Wikisource , would not arise at Wikipedia, because it doesn't try to do somethings that are encountered more often on Wikisource. One issue I encountered was that currently ProofreadPage's managment strategy for tracking things directly in page text isn't ideal. A technical meeting focusing on Wikisource would enable the relevant concerns to be expressed to developers, and a "better" content model developed.
I have some other related "ehanchements" related to Wikisource that I'd also like to talk with Mediawiki/Wikimedia developers about. Like for example, it's not currently straightforward to pull content such as quote directly from page text cross-wiki. Wikisource supports LST internally between Wikisources, but not necesarily with other Wikis. (Aside: Also being able to use certain templates cross-wiki would be immensely useful beyond wikisource. It would mean that certain projects could pull a template from say Commons without needing to have a local copy, even if the media was still locally hosted on a specfic project)~~~~