They were on the verge of bankruptcy, really the 2 options were
Let T-Mobile (a distant third competitor to the big 2) buy them
Let sprint die, the big 2 buy large chucks of sprint anyways for pennies on the dollar post-bankruptcy and make their distance from T-Mobile even bigger.
If you need another reason, AT&T was very against the deal, so you KNOW what they think is bad is probably actually good for consumers