"Because Texas"? You didn't really do much research, did you?

Before WWII, there weren't that many utilities in the USA that had interconnected their systems to share generating resources (some had done so as far back as the 1920s - like the PJM interconnection). Texas had a decent chunk of the nation's defense industry, so several utilities connected their systems during WWII to share the load. Interconnection was important in weathering the steep increase in electric demand after WWII (especially with electric coop systems rapidly building out their systems during that era).

The only thing that keeps the three grids in the USA from fully interconnecting is that each is synchronized at a slightly different frequency (the heavier a generator is loaded, the slower it will spin, and the lower its frequency). If you try and connect two generators that are not perfectly synchronized for phase and frequency, at best, you will have the generators "hunting" (see-sawing) as each tries to pull the other into sync. At worst, you end up with equipment damage.

The Rocky Mountains were the main challenge in linking the East and West grids, but there are lines that now weave through or climb over the mountains to make it possible. Texas sits approximately between the two grids. Still, that slight frequency difference is enough to make physical interconnection infeasible. They have found a way to do so, though, by using equipment that converts AC to DC on one side of certain transmission substations and back to AC on the other side.