Hi Brandon,
That is the list of apps that were connectable to via Fusion 1.0. You're most likely on 2.0, and the list of built-in connectors for your scenarios is here: https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FWo...
That being said, there are also quite a few in that list that don't directly appear in the graphical selection interface even in Fusion 2.0. If you type them in the search box though, you'll be able to add them to your scenario anyway. Ex, Bynder DAM appears if I search but not if I scan the icon list.
As to the 'why' of that particular quirk, that one I don't have any insight into.
Katherine