Probably because the developer didn't mark the Pixel phones as compatible. It might because there isn't a way to differentiate between the phones on the Verizon Wireless network and those on a different carrier.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
We got our Pixel phones in November, and enjoyed NFL games on them all season long - the app definitely works on the Pixel for us. Did you buy the phone from Google or Verizon? We bought ours from a corporate Verizon store.