Why don't they show the Packer games if the Vikings are not playing or are on a competing station anymore?

I have bet going and I need an answer. Any help would be great. I have lived in the Twin Cities for 10 years when I first moved here the Packers would always be on if the Vikings were on a different station or played later. Say the Vikings were at home on Fox 9 and the Packers had a home game against an AFC opponent they would on ABC. But with in the last 2 years, either no game would be shown or a different game. I don't remember hearing of a NFL broadcasting rules change. Thanks.