Why are graphics coordinates measured from the upper left?

It's an old convention, and the reasons might be a bit apocryphal. Here are some hypotheses I've found:

It's derived from CRT electron beam sweep behavior.

Scanning from top to bottom means you don't have to wait for an entire frame to be sent first, you just begin scanning as soon as you begin receiving data. (Which raises the question again, why scan from top to bottom)

It allows a right-handed coordinate system with the Z axis going into the screen rather than coming out of it.

Annoyingly, Cocoa and Quartz use lower-left origin.