Why is Call of Duty so popular?

This explanation attributes the franchise's popularity to depth: a set of interacting systems that takes a long time to master, so that more hours still make a player better and the game holds its audience for years rather than weeks.

This explanation attributes the franchise's popularity to speed: short rounds and quick engagements that fit the time people actually have, with a loop tight enough that attention is rarely given a gap in which to wander.

This explanation attributes the franchise's popularity to accessibility: controls and objectives a newcomer grasps within minutes, so anyone can be included in a session and play is legible from the first one.