It always was Composite video in the old days,as it is a more rugged format than the others,needs only one cable/link channel,& everything is designed for it.
Wasn't it all Betacam in those days? They stored video as separate luma/chroma.
Only inside the machines/tapes-----the workhorse signal format was
always Composite in TV Broadcasting.
Domestic & Quasi-Pro stuff played around with different formats a bit,but Broadcasting requires precise timing,& using multiple cables everywhere has the inherent risk of different time delays between the two component signals,unless each one of a cable pair is exactly the same length.
Multiply this by the thousands of Coax runs in a TV Studio!
The other formats could be derived from Composite if you needed them.
At TVW7, the AVA-Graphics suite used RGB for the Pix Monitors,& probably internally ,but the suite output was Composite.
The Studio Cameras did use various versions of Component Video internally,but what appeared as "Camera Output" to the Vision Switcher input was also always Composite.
Note on Betacams, etc:-Where I worked,Betacam was used by the News Dept mainly--The large VTRs were mainly reel to reel Sony Helical scan,with some similar Bosch units,& an old Quad scan cartridge unit for Commercials.
Later,Panasonic Digital Videocassette machines (Large Broadcast format) were used.