love that picture.

that fucking picture... oh lawd

Because one of the producers that pushed the idea of Derpy being called "Derpy" canonically actually has a retarded child.

There's nothing canon about her name prior to that point. And even if it was, you didn't debunk what I just said.
And because the fandom named her Derpy (and that was even disputed, as some went by Ditzy Doo), the creators decided to give her the name Derpy as a shout out. That resulted in the shit storm we can all very well remember. The reason that they caved and removed the name from the official iTunes release was a combination of bad press and the reason I mentioned.

I dunno.... you seem rather upset.