>>37844
>It's because that was the Spanish flag during its age as a superpower. Spain, Austria, and Bohemia were all Burgundian realms in practice. Charles V even spent most of his time in the Netherlands, not Spain or modern Germany.

So Burgundy (!) ruled them all?

https://en.wikipedia.org/wiki/Cross_of_Burgundy

The Cross of Burgundy (Spanish: Cruz de Borgoña; Aspa de Borgoña) or the Cross of Saint Andrew