Why do you think they never call Derpy by name anymore?

I hear that in "The last roundup" Derpy was ACTUALLY called "Derpy". Moms everywhere did not approve. Therefore, they never call her by her name anymore.

Derpy made her very first aperence in the piolet episode. Derpy was supposed to be a mistake that they would edit out later but they didt and that how the Derpy fanbase started, she was unique because she had as people call it Derp eyes so people called her Derpy she was one of the most popular background ponies. now for her liking muffins that from the episode "Applebuck season" when Applejack makes muffins with pinkie pie and the ackground ponyies come up and just bearly you cn hear Derpy say "Mufins" and sense then people assumed muffins were her favorite food.
why people think her job as a mailmare is from the episode "Feeling Pinkie Keen" when things kept falling on Twilight and it looks up to be Derpy and few others at a delivery truck.
now why people never call her Derpy was becaus eof an episode by Amy Keating Rogers.
now Derpys popularity was rising to the top and everybody who worked for he show called her Derpy so Amy did a scene with Rainbow Dash and Derpy in the episode "The Last Roundup" now when it aired she got hundreds of fan emails saying they LOVED IT!! but then it all went wrong. she recieved ten. just ten hate mail by mothers who had children with disability's who watched the show and they called Derpy "Offensive". Amy actually was rather hurt by that because she also had a son with disability's, so she changed the scene by having a different voice, a different name, and worst of all editing her eyes.
many fans were hurt because Amy was trying to connect with the fanbase with the original scene and now that cant happen.
but she still is a background pony in season 1, 2, and some parts of 3. we havent seen any of her in season 4 yet and fans are getting worried.
now people either call her Derpy, Ditsy Doo, and Bubbles. but her preferred name is Derpy

Likely because some say it's offensive to those with disabilities. She's still cute though, don't know why some people are being sensitive about it...

People got angered because they thought it was offensive to retards

Ditzy Doo was actually not her first name. She appeared in the background of the pilot episode, and had a speaking role in the last roundup. In the ORIGINAL episode, she was called Derpy. Then people got mad, and complained about fan service and called her offensive and stuff, so now the iTunes version has her as Ditzy.