Here's why:
* Marcia Wallace, the original voice actress for Edna, sadly passed away in 2013. The show has chosen to honor her memory by not replacing her character.
* The character of Edna Krabappel was written out of the show after Marcia Wallace's passing. While Edna's absence is acknowledged, the show has focused on other characters and storylines.
* The Simpsons has a history of respecting the legacies of its original voice actors. For example, after the passing of Harry Shearer (the voice of Mr. Burns, Ned Flanders, and others), the show has kept those characters but haven't replaced them with new actors.
While some fans might wish to see Edna return, it's highly unlikely the show will bring back the character with a new voice actress. It's important to remember that the show is honoring Marcia Wallace's legacy by not replacing her iconic character.