You have to realize the Patriots originally thought Deion Branch was going to be a member of that 2006 Patriots squad, and it would have been nice if they brought in better receiving options for Brady but they did at least try.
NE was interested in trading for Stallworth but the Saints wanted a 3rd/4th and one of the Patriots backup LB's (I believe it was Banta-Cain) NE thought that was to rich, especially since Stallworth might have split after 2006. It would have been nice to have him last year, though.
They brought in Doug Gabriel who some believed a young developing WR. It turned out Gabriel had an attitude so he found the door.
NE drafts Chad Jackson who some thought was the best WR in the 2006 draft (I still believe he could be) but he goes down with injuries missing most of training camp, and then struggled to get on the field and be effective throughout the 2006 season.
As you mentioned they brought in Donald Hayes who was supposed to be a good WR, and he did nothing. NE also tried to bring in Derrick Mason but he chose the Ravens. They also brought in David Terrell hoping he and Brady would recapture that magic they shared at Michigan....
So, it's been a combination of things. Really, I think the FO has tried to bring in good receiving options in the past but they didn't make it their main priority. This year after dropping that game to the Colts it was obvious Brady needed better weapons so they made it a priority. This time going after Stallworth (and sealing the deal), pulling Welker away from the Dolphins, and gaining Randy Moss from the Raiders for only a 4th round pick.
The other thing to think about is if Randy was available for a 4th round pick in 2006 do you think NE would have pulled the trigger? I sure do, but he wasn't available for a 4th rounder last year which is one of the key differences between the two offseasons.