Popcorn. Piles of the stuff, fifty feet high, for as far as the eye can see in any direction.
IF he can produce another cup run? And what possible guarantee can their be of that? Your opponents in a cup run are determined by blind luck. On the other hand, your opponents in the league are certain, therefore your performance in the league is a better barometer of your true level of ability. Prospective players consider that when deciding whether to join the club, amongst other things.
I appreciate that he'd have a better squad to work with here and, almost unbelievably, more money. But even you're pointing out that he'd likely do worse in the league than Moyes has. The odds of a good cup run are reliant in large part on the draw. And that's not a good enough reason to pay more for Martinez than you would have to for Pereira or Girard.