Never heard about "Digital Signature Services (DSS), FLUX TL, midPoint, WSO2". Why were they chosen?
As others have said, they are most likely being used by the EU in some parts of their infrastructure. Then the question becomes "Why were they using these software in their infrastructure?" The answer to that is probably along the lines of "a guy that was assigned to project used it because it came up in google," if I were to hazard a guess.
Isn't this how most software is chosen? Someone finds it, realises it's useful, recommends it further, ... And down the line it's an essential tool.