Nexstar weren't impatient. They moved quickly after the FCC approval probably anticipating State Plaintiffs/DIRECTV will file a TRO but also because Bank of America gave Nexstar a high-interest 7-day bridge loan to close the deal. As soon as the approval came TGNA was removed from trading and Nexstar moved quickly to sell bonds to repay the BoA bridge, and pay TGNA stock holders their $22/share. These deals are loaded with tens of millions of $ in fees so any delays end up costing.
Asst. AG from the State Plaintiffs complained in the filing that she emailed Nexstar counsel to delay the closing, but they weren't responding for something like a week, the merger closed, and Nexstar replied saying it's too late to wait. Not illegal, just State Plaintiffs got outmaneuvered. Nexstar is not required to wait or accomodate any pending litigation unless it's court ordered, it's on the plaintiff to counter it.