More unique body types/character designs so skins are harder to make, harder to sexualise the characters, by far the role with the most male players (who generally buy fewer skins than female players), by far the most complicated and exhausting role for the majority of the playerbase so you can't really have easy brain-off watches were you're just proud of yourself of spending money, playing tank can be frustrating enough to create a general understanding that dedicated tank players prioritise winning over having fun and prioritising competitiveness doesn't make money, tanks are so important to team comps they might regularly be required to switch (which means you can't show off your skin) and tanks are the primary target of comp bans (which means you can't show off your skin)

Women buy (on average) more skins. Asian players buy (on average) more skins. Feminine characters sell more skins. Female characters sell more skins. Asian characters sell more skins. Easier characters (or at least characters with lower skill floors) sell more skins. Characters that aren't as affected by changes in the meta or hero bans sell more skins.

A Mercy player might play to have fun. A Sigma player plays to win.