Spiner had to leave Supergirl due to a family issue that conflicted with production dates.
Babylon 5 alum Bruce Boxleitner has signed on to play a role originally slated for Brent Spiner, who is undoubtedly best known for his work as Data in Star Trek: The Next Generation and a number of Star Trek films.
Rather than write the character out until a time when Spiner could jump in and bring the role to life, Supergirl decided to recast the role, which points toward this character becoming quite important over the course of Season 4.