Really Hub should realize that this cartoon has a fan-base older than six and that they should think about what they would like too. Not saying they should ignore the child audience but still.
...They do. That's why Derpy showed up in an episode to begin with and why Derpy was this year's SDCC exclusive. However, while the adult fanbase exists and is very vocal, it is dwarfed in size by the show's target audience. Who are reliant on their parents to buy them all the stuff (toys, coloring books, DVDs, stuffed animals, pillows, bedding, storybooks, dress up outfits, software tie-ins, backpacks, school supplies, toiletries, band aids, fruit snacks, trading cards, wrapping paper, etc, etc, etc.) that Hasbro wants to sell them. Whose parents complained about the character of Derpy, which is why she is/might be underped Ditzy Doo in all future speaking appearances.
Hasbro is making the only right call. The adult audience is fun, but we just don't buy the same amount of licensed crap that parents will. Little kids will immerse themselves in their favorite show*, adults will mostly just watch it.
*For instance, when I was a kid, I had Powers Rangers licensed: action figures, bed sheets, shoes, Halloween costumes (2 out of the 5 years I watched the show), books, trading cards, band-aids, comic books, audio cassettes, Tiger electronics video game, sleeping bag, stuffed animal, watch, and big wheel. So yeah.