So Idris Elba actually isn't a liar. Back before Age Of Ultron hit screens, the British actor had accidentally announced that he and Tom Hiddleston had filmed together, revealing their cameos with flagrant disregard for Marvel's secrecy rules. In retrospect, it didn't really matter that anyone knew they were involved, because their sequence was no more than a nightmare vision used to haunt Thor. But even in that limited sequence, it seems that Hiddleston's performance was just too good for test audiences, who inevitably heaped more emphasis on his presence than was actually intended. They couldn't disassociate him from the plot, and things got too complex, which definitely wouldn't have happened if he'd just phoned it in. Here's how he related that to Digital Spy:
"I was part of the dream sequence for the character of Thor. I shot for a day, and I enjoyed it very much, and then I received a phone call from Kevin Feige...who said, basically, in test screenings, audiences had sort of overemphasized Loki's role. So they thought that because I was in it, I was controlling Ultron, and that it was actually imbalancing peoples' expectations."
Despite some suggestions that he might want to move away from the MCU, Hiddleston wasn't bitter about the cut:
"It made sense to me when I saw the film. Ultron was the bad guy. That was important. And that's why Loki wasn't in it."
So, who are these idiot test screeners who believe that an obvious dream sequence contains as major a narrative thrust as an entirely different villain in control? What about that sequence made them all abandon the ability to evaluate obvious fantasy? Well, there is something of an answer, if you choose to believe they weren't all just idiots. In almost all of the cases of the Avengers who see visions, they are treated to their worst fears (including Black Widow, who already lived hers), but for Thor, somehow he sees a flash of a future that will actually come to pass. It's not as if Scarlet Witch just incapacitates him with fear, she somehow manages to open a grotesquely mangled window to the actual future. It doesn't really make sense, frankly, BUT you can see why the audience might have taken Loki's presence as something more in that context. Still, it's a shame Hiddleston was cut.