I think there are multiple reasons for shelfwarmers.

From what I've observed certain waves of toys (depending on store) are larger or smaller then others. For example HFTD Payload is in far greater supply then RTS Jazz where only 2 made it to the shelves (Target)

Currently Ejector and Scalpel are shelfwarming in stores alongside Wave 1 PCC 5 packs, HFTD and ROTF Voyagers though it appears the the biggest type of toys are repaints.

I can't say whether the stores, supplier or customer are to blame for shelf warmers. I can't help but wonder if the money spent on repaints that don't sell to save money couldn't be better spent on new moulds that could sell.

Can't Hasbro see that the money saved on repaints is going down the drain in unsold product?