We saw a Marcus in the prime universe - he just happened to be named Cartwright, and wasn't the Big Boss yet*. We got another one a century later. Starfleet probably sprouts those almost as often as it kills space monsters... And the butterfly effect just alters the names and faces.
* Cartwright was a five-star Admiral, too, if we trust the Fletcher scheme. But he wasn't the only one, or the first among equals. Leyton was two rungs down from there. I wonder what the top of the organization chart really looks like?