Wheat is the simple-starch-rich seed kernel of a hybridised grass.

Much of Texas lacks water -- generally West Texas, a line extending from roughly Austin. In the US generally, the 100th parallel divides the moister east from the drier west. Topsoils are also far poorer than further east.

There was an experiment conducted some years ago on dryland farming. It ended poorly:

https://en.wikipedia.org/wiki/Dust_Bowl

Timothy Egan's The Worst Hard Time is an excellent telling of this:

https://books.google.com/books?id=np1RwDQfpjsC&pg=PP1#v=onep...

Not all grasses are equally edible. Most aren't, in fact.

> Why?

Not enough water.