When we last saw Tim Blake Nelson's Dr. Samuel Sterns--and minor spoilers for The Incredible Hulk--he injected Tim Roth's Emil Blonsky with the blood of The Hulk, which led to his own experience with the blood of Bruce Banner. One final shot of Nelson's head starting to transform was all we got, and like that, the villain known as The Leader was born. So why haven't we seen him rear his extended cranium yet again in the Marvel Cinematic Universe? Well, Marvel Studios president Kevin Feige has an answer to that question, and it goes something like this:
It is an embarrassment of riches of places we could go or characters we could reprise or bring back now that we're... I think Ragnarok is our seventeenth MCU film. And you would be surprised how many... it's come up. It's come up. Just finding the right place. And as I said, if you can't do it right, don't do it or do it later, and the notion of 'Hey, I'm here too! Next!' is not interesting to us. Abomination is in a prison somewhere too.
It's hard to argue with Feige's claim that Marvel Comics has such a rich catalog of heroes and villains that it's kind of difficult to revisit anyone without a good enough excuse. The problem isn't that The Leader is a character that Kevin Feige would rather forget about, but instead there are just too many Marvel Comics villains that haven't been used in the MCU canon. Why bring back a super-powered villain folks hardly remember, when there are so many menacing baddies that are waiting to make their own unique impressions on audiences worldwide?
Though it also probably doesn't help that the rights issues that persist between Marvel Studios and Universal Studios still effect the possibility of any sort of solo Hulk follow-ups. As Feige also discussed with Crave Online, the fact that The Incredible Hulk's particularly unique legal situation has also prevented any sort of return to not only Tim Blake Nelson's super-powered persuader, but also Tim Roth's jacked-up Hulk wanna-be. Having a villain bench as deep as Marvel Comics' rogue's gallery does help when trying to keep an empire going. And just as Kevin Feige has admitted, it helps avoid revisiting certain villains too often through the various phases of the MCU, though don't tell that to Tom Hiddleston's Loki.
With that in mind, there is a canon explanation as to The Leader's current whereabouts, and it's all thanks to The Avengers' movie tie-in entitled "Avengers' Prelude: Fury's Big Week". Apparently, thanks to a post-transformation run in with Natasha "Black Widow" Romanoff, the man formerly known as Dr. Samuel Sterns is now officially in S.H.I.E.L.D. custody. Though it doesn't help that he ended up in such a situation after trying to win Black Widow into his own employ, a gambit that resulted in Romanoff shooting Sterns in the leg to subdue him. So unless Avengers: Infinity War, or any other future MCU film calls for one super smart super villain to be released from the clutches of Agent Coulson's finest, we doubt we'll be seeing him anytime soon.