Yeah I don't know either, maybe they figured that they may as well keep the same showrunner through the end of the series & finish it off that way? If they wanted to switch gears & go with a new showrunner they should have done it after the 1st or 2nd Goldberg/Chambliss seasons but it seems they missed that chance. At this point maybe they're too far in to change things up especially since this is the last season anyway.. that's my theory.
You sort of see the same thing happen with the main The Walking Dead series but at least it worked out better there. They switched showrunners & Angela Kang did that for the last 3 seasons. Sure some people still complained but I'd argue that she did get the series back onto the main plot & kept the story moving towards the finale.
Thankfully the other shows are much better overall (Dead City and Daryl Dixon).