Complex sentence: They needed a standardized bus for communicating device abilities between monitors and computers, and selected I²C because it required only two pins; by re-using existing "reserved" pins in the standard VGA they could implement a complete A.b bus (including power).
Split into simpler sentences: They needed a standardized bus for communicating device abilities between monitors and computers, and selected I²C because it required only two pins. Re-using existing "reserved" pins in the standard VGA connector allows for a complete A.b implementation (including power).