I think... for narrative purposes... if Keith's dad was still alive there would be no character motive for him to build a relationship with Shiro and then pursue him when he disappeared. It's like when they kill moms in manga/anime, because a parent is a strong, centralizing figure. With no attachments, Keith is free to be manipulated by the narrative.
I would have rather had his dad be in a coma? But then we run into the same problem. Would Keith leave the Earth if there was a chance his father would wake up?
Maybe have his dad be a pilot too and disappear on the same mission, would have worked? IDK. The writers were very lazy at times...
Here's a cute baby Lotor.