I don’t have any idea really, but one possible contributing element is the speed of delivery. My understanding (possibly incorrectly) is that western comics are more commonly delivered on a one book per month cycle, whereas Manga delivers a lot more content in the same time period. Part of this production time can probably be attributed to coloring time (Western comics color every panel vs Manga printed in mostly black and white).

There’s also the accessibility of Manga and anime, having relatively newer characters without the burden of decades of backstory (not accounting for One Piece). Running an anime with (mostly) similar story line helps to bring potential new readers up to speed quickly with Manga, whereas the animated adaptations of western comics often seem to pick specific story arcs of comics, or make up entirely new stories.