Skype has a long history and widespread adoption, but has had a lot of issues with quality and connection over the past several years, easily being eclipsed by most of the other tools we mentioned.
We're not really in a position to provide an exhaustive list of potential platforms, we simply picked the ones that are the most prominent right now and/or are tailored specifically for gaming in some way. For most users, a more reliable, concise list is likely more useful.
But we also are permitting event organizers to submit events on almost any platform they would like to use, though we of course reserve the right to review and adjust individual events as necessary.
If Skype has worked great for you and your players and that is your preference, you are absolutely welcome to run your games through it. It's simply not an approach we recommend broadly.
Senior Event & Program Manager
Gen Con LLC