Here is the explanation I found online
"For radio broadcasts, the NFL follows a nearly identical policy to MLB. There are no radio blackouts, but only each team's flagship station can carry local broadcasts during the conference championships or Super Bowl. All other markets must carry the NFL on Westwood One feed for those games. For all other weeks, within 75 miles of a team's stadium, only stations the team or its flagship station contracts with can carry those games, regardless if the team is home or away. Thus, any competing station that carries Westwood One broadcasts cannot air those games"