DISH has been quietly collecting spectrum over the last few years. The plan was for them to enter the wireless market, however that still has not happened. And soon they'll need to figure out something to do with their spectrum. Either deploy it, or license it to other wireless carriers. Something that Comcast and Charter did with Verizon a few years ago - hence their MVNO agreement with Verizon, that they are activating next year for their own wireless offering. BTIG research seems to think that any of the top three carriers (that's T-Mobile, AT&T and Verizon) would be interested in picking up DISH's spectrum.
When it comes to DISH's spectrum, they are mostly in two bands. There's Band 66 which has 20MHz of the 40MHz of AWS-4 spectrum. Right now that spectrum is valued at $1.83/MHz/POP. Meanwhile the other band is Band 70 which includes the remaining 20MHz of AWS-4 spectrum which they obtained from two bankruptcies. They also have 15MHz of uplink-only spectrum that they got from the AWS-3 auction and then there is 5MHz of downlink airwaves from the H-Block auction that took place back in 2014. Band 70 is valued at about $1.26/MHz/POP, so it's slightly less valuable than their Band 66 spectrum.
This spectrum is what is known as "high-band" spectrum. With Band 66 being 2.2GHz and Band 77 being 2.6GHz, they are going to be good for 5G, because it can handle more bandwidth than the low-band spectrum that carriers were going after pretty heavily with 4G LTE. This spectrum is valuable, especially for carriers, and if DISH does ever realize their dream of offering up a wireless service, this is the spectrum they'll want to start off with - especially if they can get, or have, an MVNO agreement with another carrier. Given that this spectrum is close to the 2.5GHz spectrum that Sprint already has, it makes plenty of sense why Sprint is the only carrier not interested in it.
BTIG also noted that it's possible that a tech company or cable operator could emerge to claim this spectrum from DISH. It's unlikely, but it could happen. Walter Piecyk from BTIG wrote, "with the proliferation of fiber and the dramatic drop in radio costs, the cost to build a new network from scratch has never been lower." Which is likely going to bring in more competition int he wireless industry.