Well, it's a combination of things really. Fanfiction writers often move on from a particular fandom or stop writing, and sometimes their works disappear as a result. In the case of 'fem Harry Potter' fanfics, they might not have been as well - received by the broader Harry Potter fandom as other types of fanfictions, so they didn't get as much attention or were not shared as widely. Also, the way fanfiction platforms manage and index their content can be a bit of a mess. For example, if a fanfic is not properly tagged with 'fem Harry Potter' related tags, it becomes very difficult to find, especially when there are thousands of other Harry Potter fanfictions on the platform.
There are several factors at play. Firstly, the Harry Potter fandom is huge, and there are countless fanfictions out there. The ones focused on a 'fem Harry Potter' concept might be overshadowed by more traditional interpretations. Secondly, some of these fanfictions might have been removed from certain platforms due to various reasons like copyright issues or violations of the platform's terms. This makes it seem like they are'missing' when in fact they may no longer be available legally. Moreover, the search algorithms on some platforms may not be optimized for finding such specific content, which further adds to the difficulty.