I don't have an issue with GB moving back. Here's the problem, the will need a trading partner that is willing to give up enough value to make it worth their while.
As has been pointed out a number of teams are looking to trade back. Several teams already have. (4 of the top 9 teams already have. Bears at 1 and Carolina at 10 should count double especially since Carolina moved up not down.) Add in the Cleveland/Houston and GB/ NYJ trades, that's a lot of movement. The number of teams willing to climb in is less. They will if they think they are getting a deal.
It's been said there are only 8-10 quality players in the first round. Are they all gone at 13? If so why move up? There are more than enough players of equal value that can be selected at the back of the first or top of the 3rd.
Then as is mentioned several teams are looking to trade back. Too many teams wanting out and too few teams willing to move up means it is a buyers market not a sellers market.