Fair question....
One reason is because of draft status. Peterman was a 5th round pick & Allen a first round pick.
Peterman was a projected back-up NFL QB, Allen a potential solid to good if not great NFL starter.
Peterman has looked bad since he started playing. Granted he was dealt a bad hand and was thrust into the spotlight when he probably shouldn't have. Those interceptions has left all Bills fans with a bad taste, and they don't want to see anymore from him. It may be unfair, but that's how it goes.
Many people feel that Allen can and will be the future of the Bills, myself included. He was given a bad deal and thrust into action immediately, when some of us wanted to see him holding a clipboard all year. That's not how it turned out and here we are.
I personally hated the Peterman pick. Even though it was a 5th round pick. I saw nothing from him in his college career that convinced me he was going to be a starter in this league. Josh Allen was not my top QB in the draft this year, but I was fine with the Bills selecting him. He's raw, but I believe that he has good to great potential.
I feel that Allen is progressing. Slowly, but progressing. I really think we would have won today, had he not gotten hurt. That pass to Benjamin was big and you could see that he was hyped and hungry and ready to go on the attack, but he had to accept that he was injured.
That's the best way I can put it. Peterman got sucked into a no-win situation and now he has been stigmatized. He threw two very nice passes to Zay Jones & Andre Holmes today and then he threw two awful interceptions. The loss goes with him, that's a bad deal, but that's how it goes sometimes.