GitHub/GitLab are for developers, SourceForge is for users, notice how one puts code up front and center while the other has a description, screenshots, categor

GitHub/GitLab are for developers, SourceForge is for users, notice how one puts code up front and center while the other has a description, screenshots, category information, user reviews, ratings, etc, the most prominent buttons are to download it and get updates about it and at the top the links are about getting the project's releases (Files), reviews, support, mailing list, tickets, news - ie. stuff that are more of interest to end users than developers - and only at the end you have source code access. Some projects do not even use SourceForge for the code repository but only to host releases, provide mailing lists, forums, etc. Of course the UX could improve but that is another issue.