The first reason is that the actor who plays Mockingbird, Adrianne Palicki, was cast in another show called The Orville. This caused a scheduling conflict, and she was not able to do both shows. The second reason is that her character, Bobbi Morse, was becoming less relevant to the plot. In the early seasons of Agents of SHIELD, Mockingbird was a key member of the team, but as the show progressed and new characters were introduced, her role became less important. The writers decided it was time to write her off the show.