Because it's been clear for years that they don't think they need to invest in the DT position. Yeah, they threw a 2nd rounder at Hill because Marinelli was in love with him. But other than that? Mid to late round picks, fringe bottom of the barrel DTs in free agency. They signed two guys (Poe, etc) last year that many thought would help but they were clearly on the downside of their careers and didnt work out.
Philly is the exact opposite. They think DT is one of the most important positions on the field and they put a lot of assets into that position. Cox was a first rounder. Davis was a first rounder last year. Hargrave was a prime FA signing in his prime. So when they were struggling to stop the run and Davis got hurt, they went out and grabbed not one, but two, guys they thought might have a little left in the tank.
And guess what? They have two first round picks next year. Don't be stunned if they use one of those picks, or even their 2nd rounder, on a DT again. They think the value and way to build a winner is through the trenches, especially at DT.