Why would private equity want to buy a giant of the dinosaur age? It isn't a viable long term investment and how much can be sold for parts?
If they really can get it for $20 billion they'll make money. Directv is still generating over $1 billion in cash flow each quarter despite the subscriber losses, so if they can maintain that it'll pay for itself in less than five years.
Maybe they can't maintain that forever (though they have so far) but they'd easily make a nice return in a decade. That's around the time when the satellites might start needing to be replaced, but the business would probably mostly be whatever "AT&T TV" is renamed to rather than an actual satellite business by then, as broadband/5G would be available to most people in the US.
As for why AT&T would be willing to sell for only $20 billion, I'm skeptical. The one thing in favor of that is that the CEO who did the purchase is retiring, so the new guy can blame the old guy for the big loss. If he was staying on he wouldn't want to take the loss on the sale himself.
AT&T has owned it for what five years now? So they've probably skimmed at least $20 billion in profit off it, and got the benefit of better programming cost for Uverse TV, so if they could get $25 billion it might be close to break even. Not that they were looking for break even when they bought it, but at this point they'd probably take it.