Like I said, maybe SHIELD just didn't know about them. Like, for example: Namor lives at the bottom of the ocean. Dr. Strange pretty much keeps to himself and does his own thing and doesn't draw a lot of attention to himself. Black Panther might still have been in the middle of training to get revenge for his father's murder. There's no reason that SHIELD would have to know about these guys already.
And even if SHIELD did know about them, or at least new something about them, there's a perfectly reasonable explanation as to why they were never mentioned: It never came up. Simple as that. If SHIELD wasn't able to get in contact with them or whatever, then there's no reason why it should have been mentioned in the movie. Those would have bee unnecessary details.
The world is ending. We're out of options. Things natural would come up. They didn't. And the more people with incredible abilities that just happen to not draw any attention to themselves, the less likely it becomes. When you start getting to a half dozen of these people it begins to feel contrived - because it is. This works for those properties as if we are ever introduced to these characters, they will most likely have origin stories, and not be portrayed to have been in operation all this time, and just conveniently absent when the world was ending.
Yeah. This is really the kind of thing that, at worst, requires a two line "explanation" once or twice in the TV show. "So if Prince Namor can lift a cruise liner, why didn't you ask his help against Loki?" "Hah, you have clearly never met Namor, agent."
Once or twice isn't too bad. It's possible everyone who was aware of, say, Namor had the exactly same perception of Namor, so there was no discussion that would naturally arise in the world's most desperate hour. I don't think that explanation would satisfy, when they clearly got Stark of whom similar could be said, but explanations could be created. But there can't be a dozen such people SHIELD is aware of and no one at any point said: "Hey, what about..."
Edit: Here's what I saw - I saw big things go down and SHIELD approach/pull in everyone that may be of any use, regardless of how dangerous or unstable or unsuitable they may be. It turns out that everyone they ask eventually accepted, fortunate for them. Then these people get taken apart and all seems lost. Fury's only play is to push who's left to act. The clear implication is that there is nothing/no one else to do. They are desperate, but this is it. No other options come up, even from the critical Maria Hill or World Security Council. Everything works out, and even when future conflicts are mentioned, the only hope is that they will return. There's no room in that desperation and comprehensive action and conversation for an alternate set of super people, imho.
Anyone they come across on the SHIELD show should be non-powered or low(non-flashy)-powered and/or not been in operation for very long at all. This just happens to work best for a TV budget anyway. They get to meet things at their outset, and run into people who can be shown in action without blowing the budget for the episode. Punishers and Iron Fists that rarely use their chi, and Dr. Strange's that aren't sorcerer supreme yet. All good bets. (unless they plan on making any of those films).
I think it'd be better to focus on emergent villains, the more obscure the better, imho, and use Misty and Colleen as the H4H as recurring cast, so that the show doesn't tread on any likely movie properties' potential narratives.
