TV Guide got to speak with showrunner Keith Eisner about the new direction the show was headed in, and he had some interesting things to say about the decision to write out Alex’s character entirely.
“…for Kirkman to be home alone, if you will. To be a widower in the White House. To be a bachelor. That will have personal and professional repercussions for the remainder of the season. And it’s also terra nova dramatically, we haven’t seen this in a television show,” Keith explained. “A president who is essentially by himself, and the isolation of being a single parent and of having to figure out what to do professionally and personally without his partner.”
Source : https://www.distractify.com/p/why-did-natascha-mcelhone-leave-designated-survivor