Why Does Texas Have Its Own Power Grid?

Because the Eastern and Western Interconnection cross state lines, they’re subject to the Federal Energy Regulatory Commission (FERC), which regulates how electricity is transferred and sold in the U.S.  
This commission was formed in 1935, under the Federal Power Act signed by FDR. It also oversees other issues related to power generation, such as hydropower, natural gas, and liquid natural gas (LNG) production.  
Texas wanted to avoid federal regulation of its power system, so it chose to keep its power grid separate from the other interconnections. That means the Texas power grid doesn’t cross state lines and can be managed independently. 

https://justenergy.com/blog/texas-power/