Because Sling was a separate company and was not associated with a TV distributor. Now Dish owns sling and Dish has a contract with FOX. Dish has put sling into Dish hardware. FOX should have no trouble saying they are suing now because things are different.
The contract aspect has zero to do with any infringement claim. By not suing right away allowing people and companies to purchase and use sling tech for so many years, they have weakened their copyright argument.