SourceForge is (and always was) more oriented towards end users, this is why when you visit a project's page you get a big fat DOWNLOAD button at the top of the page, a user score, buttons to be notified for updates, etc, when it was last updated, followed by a summary about what the project is, what its releases are, the categories it belongs to, its license and then UI to rate the project, comments by users, etc and the main buttons are about said summary, files to download, more reviews, support, mailing lists, forums, wiki, news (some projects have more or less, depending on the configuration) and at the end access to source code repository. At the sidebar there are even related projects and places where you may find other projects in case the one you are looking at doesn't meet your requirements.
For developers obviously only a small part of that is relevant but the service is not really primarily for developers and since developers "hold the keys" it makes sense that once services that care more about developers appeared many developers moved there. From a user's perspective it is sad though since pretty much every other supposed alternative does not provide a lick of SourceForge's features and the fact that there isn't any after all these years shows -IMO- how much most FLOSS developers care about their users.
On the other hand SourceForge always had a terrible UI, especially in its earlier days and while its current incarnation is the best its ever been (aside from the oversized elements) it still often feels like a mess of elements jumbled together and while they do provide a lot of useful functionality, it is often behind very clunky interfaces.
I wish there was a real alternative though.