I guess that's a repercussion from the succession. It was a long time ago, but I'm sure there were many people in positions of power that still held those viewpoints and wanted to keep everything separate for when that (in their minds) inevitable day comes again.
My understanding is that it is a side effect of WWII. Texas had a large amount of plants needing redundancy. Afterwards, ERCOT's independence allowed the state to bypass a significant amount of FERC oversight and regulation.