Because they made a minimal effort and hoped for the best. They lost their three best starting OL from a group that was marginal to begin with. In response they signed a mediocre center and a backup tackle, and drafted a developmental guard prospect in the 5th round.
So the plan was apparently that Dawkins replaces Glenn, Miller/Newhouse/Groy replace Incognito and Bodine/Groy replace Wood. And they just keep on rolling with Ducasse and Mills, their two worst lineman from the previous season.
How could anything go wrong?