At least part of the reason is regulatory. The national grid regulator (FERC) gets its foothold through the interstate commerce clause. Since Texas heavily rest

At least part of the reason is regulatory. The national grid regulator (FERC) gets its foothold through the interstate commerce clause. Since Texas heavily restricts the interstate interconnections it is not regulated federally (or at least less regulated). Of course this doesn’t explain the East/west non-interconnection, I think historically the Rockies do most of that though (as the article mentions). The issue is only partially technical and largely administrative.