Because they take every other player who the Packers decided was too old to be worth what they were demanding, so why not Jennings?
This is nothing new. The Green Bay Packers have let the contract of a good player expire because the two sides do not see eye to eye on money. While the Packers still think the player -- in this case Greg Jennings -- is a good player, the team feels that the player has reached an age where they are not worth the money they are demanding over the number of years that they are demanding it. In this scenario, said player occasionally goes to the Minnesota Vikings, who are willing to pay up.
Greg Jennings, you're the next contestant on 'Packers castaway in Minnesota'! The Packers' wide receiver has scheduled a visit with the Vikings, who are in need of some wide receiver help after trading Percy Harvin, who was probably a bit overpaid and who criticized Christian Ponder. They have money to spend, and could very well offer Jennings more than the Packers are willing to pay him.
Of course, we have no idea whether or not the Packers' old offer of $10 million per season still stands, or whether the Vikings think he's worth that much. Given that Danny Amendola and Wes Welker both went for $6 million per season, it's tough to imagine that the 30-year-old Jennings, who has been injured in each of the last two seasons, can command Mike Wallace money.