I think that the 3 languages were chosen within the languages that the current developers of the feature can understand, so that they can rapidly work on the submitted feedbacks.
For Persian users, this would mean experimenting first in the English Wikipedia, or in Meta where it could be tested as well, before going to add some RTL languages (like Hebrew, Arabic and Persian) and fix some RTL layout issues in the UI, and then add one of these RTL wikis for tests, and then extending the test on Commons, and later on all wikis. Some languages require additional developments because of their script, or because they use local script variants (and need support for the Wikimedia transliterator integrated on these wikis, such as Chinese, Kurdish or Serbian).