I think another key to this run's popularity (and this may be controversial) is that it provides a few mysteries. Throughout the series there's been a few popular takes (for traditional non-Hulk fans) and the one consistent thing with those takes has been a mystery.
Bruce Jones - Mr. Blue
Jason Aaron - Hulk/Banner split; Evil Banner
Mark Waid - Who Killed Bruce?
Loeb - Red Hulk; Ross's "death"
Inevitably, the new fans that were brought in, went away. Whether it was the mystery being revealled or dragged out too long.
Another key "formula" is change. The title has always been about that. PAD continually reinvented the character. Pak did the same. It was less about the fights and more about new directions they could pull the character through. It's why the character worked with Avengers because it was different from before.
With Ewing's take, there's multiple mysteries (for new fans) reinventions (for long term fans).
I mean, it's basic storytelling but it's interesting to look back and see how that's affected the title over the years. Why it suddenly gets popular and then wanes once the mystery is over.
Similarly, it goes a long way to explain why when PAD and Pak revisited the title, the "magic" was gone.. fans don't necessarily want a greatest hits collection so to speak (at least from my experience) PAD's 2nd run (Tempus Fugit wasn't a new mystery, but rather retconning things)