Ross Ridge's comment is correct:
I believe it's essentially determined by the media the the manga is original published in. Just like how the content of the manga needs to be age appropriate to for the target demographic, so does the use of furigana. Any one not in their teens is going to have difficulty reading manga without it.
Each manga magazine has target audience. It's usually categorized into 6 sections, by ages and gender: 少年 (boys), 少女 (girls), 青年 (youngs), レディース (ladies), 女性 (female), 男性 (male).
Boys and girls are "officially" targeted for elementary school students. Youngs and ladies are for the teen.
For boys or girls category, like Jump, "magazine" or Sunday, they always use furigana. Publisher enforces it and doesn't allow furigana in furigana (example: using kanji 二律背反 as furigana to ジレンマ (dilemma), because the kanji also needs furigana). In your example, Air Gear and Attack on Titan came from "magazine", they always have furigana.
For Youngs category, they use furigana for difficult kanji only or the first time it's used. Kaiji is published in Young Magazine and Mushishi is published in Afternoon and both are ok to not include furigana.