Us fans might buy evil characters as collectors over the long term and of multiple toys... but we are supposed to be a very small portion of Hasbro sales.

Parents on the otherhand, who make up the bulk of Hasbro sales, prefer to buy hero toys... so if there were equal number of good and evil toys, then the evil ones will shelfwarm and clog the shelves.

If you were a parent or relative of a kid and are buying a random action figure toy as a gift, you will more likely get something you are familiar with that you think they are interested in or could be interested in... and most kids want the hero toys, so parents are more likely to buy them for them.
After all, how many kids want a Lex Luthor toy before they get a Superman toy, or a Joker toy before a Batman toy... which is why there are at least a dozen super hero toys to every villain toy in a toyline.
Anything more balanced to the ratio and Retailers are stuck with toys and it kills off the toyline for not being re-stocked by stores.