In general, it's not just an attitude toward "girls toys" and "what girls want".
When you walk into a toy store MLP are not the only girls toys that have been made to be sold. While the only direct competition would be other toy horses, they have to vie for your buy against Barbie, Shopkins, Enchantimals, and all other toys, which is even more difficult now that society has started to relax on gender restrictions. That's a key factor when it comes to marketing- you are literally one of a million toy lines, so "standing out" is difficult to do when you want to ensure the longevity and profitability of the line you are selling.