There's about a billion reasons why. Just the pipeline (without accounting for pumping infrastructure) is around $4 million a mile (probably closer to $5 million these days). Materials, construction, land acquisition, regulatory costs, etc. It's around 225 miles between Lavon and Travis. So, there you go, around $1 billion just for the pipe. Then add a couple hundred million for pumping infrastructure, then add pumping costs when it's transferring water, which it can only do every so often (as you noted....they haven't had so much water that they've needed to open gates for 13 years). So in between, that pipeline is just sitting there, gathering dust.
Transporting large volumes of water long distances (anything over 20 miles or so) is a ginormous, very, very expensive proposition. And people scream if their water rate goes up 5%. So....nope, not gonna happen.
Eventually, Texas is going to have to undertake some REALLY expensive options....because we avoided those options when they were more affordable.