It's probably got to do with the bizarre design that hides the actual link to a productms website as a small element on the page and makes the most prominent links and buttons go to other things instead.

They're not very good at identifying what's actually an "alternative". By way of example, some of their top alternatives to OwnCloud include Dropbox, MEGA, and Google Drive. (All of which completely miss the point - OwnCloud isn't just a file storage service, and one of its key features is that it's self-hosted.)

I really like alternativeto. It's not always good: sometimes there are simply no good alternatives, or the community hasn't voted for the ones I'd have voted for and so a good option is way, way down. But if I want to know alternatives, I go there directly, so I guess that's maybe why people block it from appearing in random search results? I found it puzzling to see a useful site blocked (especially when I haven't seen it appear much in search results, but then, I've also been using DDG primarily, which ranks things rather differently)