NFL rules prohibit another game from being broadcast when the local team is playing at home. The theory being that they don't want to entice you in to staying at home to watch a better game. Getting your money for the ticket is not enough; they want to make sure you're at the stadium spending more money too.
Unfortunately for me, the year I didn't opt for Sunday Ticket - figuring the Pats would be on TV so often after last season that it wasn't worth getting - this is the second or third time my local station can't show a Pats game, because Tampa Bay was playing at home at the same time. It's also the second time I get no Pats on TV because of the flex scheduling; TB - SD was originally a Sunday nighter but got flexed out, just like the Pats did a couple weeks ago.
Fortunately for me there's a Pats bar about fifteen minutes away, so that's where I shall be once again on Sunday!