so what’s the reason they’re the only state independent from the national grid?

en.wikipedia.org/wiki/Texas_Interconnection

There’s a whole article on it in Wikipedia, but the TL;DR; boils down to “If we’re not connected nationally then we don’t have to abide by national regulations”.

That’s ostensibly a cost-saving, assuming you don’t think too hard about what’s being regulated. But its also a great opportunity to price gouge consumers.