The thing about Gillen's run is that it was never supposed to be an ongoing story. He just wanted to do one big arc, so he picked the few characters whom he thought the arc should be mostly about and ran with it. His decision to highlight Loki wasn't shocking, as the story felt like a spiritual continuation of his Journey Into Mystery arc. The book was very important to establish past, present and future for that version of Loki. And as for the Young Avengers, I think he was invested in the Billy/Teddy pairing more than anything else, as a lot of the book revolved around their relationship, from their first fight to their love saving the day; it was pretty much full circle. Everyone else was just kind of... there.
I get why people disliked that, but I feel like Marvel could've easily fixed that by just letting someone else take over the book immediately after he left, since it was finished and not cancelled. We could've had other arcs that focused on other characters, specially since Loki had left the team at the end of his run. But editorial decided to completely shelf the title for some reason and now we're here, 6 years later... still waiting for a new book. Honestly, it's kinda like a curse for YA fans. Same thing happened after Volume 1; it wasn't cancelled, but Marvel waited years to continue the story because Allan Heinberg was busy. They could've let other people write ongoing YA stories during that time, but no, they wouldn't let anyone other than him write it. So it took him 7 years to write just 23 issues of stories, and some other stuff that he had planned never even saw the light of day. YA fans deserve an award for being so fucking patient.