As far as why, it’s because the Explore page was written a long time ago in now-outdated code, and rather than add improvements to it over time, such as when annotations were added to the website, the staff have been waiting to just completely rewrite the page. You can weigh in with ideas for that redesign here: https://forum.inaturalist.org/t/ideas-for-a-revamped-explore-observations-search-page/8439 (filtering by annotations is already on the list)
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