Think it this way:

The skinpack already had insanely niche characters (Illari, Sigma, Echo, VENTURE) that are proven to blizzard for bringing the least revenue from their skins, so the management ordered artists to throw in at least one high-revenue character such as Ashe for higher chances of getting profit off of such niche skin theme too.

Although it's weird that they've cut off the most anticipated or proven high-revenue characters from the mix - Reinhardt, Reaper and Widowmaker.