They are in an unenviable position. Doing nothing is not an option, they have to run somebody. If they were to count it as a loss it'd be an open admission of weakness which would signal the Progs and RINOs to pounce. Keep in mind, the RINOs aren't merely controlled opposition, they were controlled by the simple fact that the Uniparty could offer them power and options through betraying their own side. They are, to a man, opportunists. The Progs are much the same, albeit less polite about it.
The DNC and Democrat Establishment, in general, need to continue to project an image of strength even as they take wound after wound. So they -must- run someone or they have to face the vultures gnawing on them at the same time an actual opposition is gunning for them. A war on two fronts. Beto doesn't require them to build a brand, and they can pretend its because he's so good. He's doomed to failure, but an easy sacrifice and the least cost for effort to fail without projecting weakness.