Because 3rd party plugins have to be adapted to work with the mobile app. Very few have had that modification yet. My Gapfill question type worked with the old version of the Mobile App and Moodle 3.5 came made it easier to make those changes. Gapfill now works with the current Mobile App as well. If anyone wants Wordselect to work with Mobile you can contact my employers Moodle Partners Titus Learning for a quote.
It would require approximately 4 days of development time including testing and making available in the plugins database.
I have a good idea how to do it as there are strong similarities with what was necessary for Gapfill so it might come in at less time than that.