They didn't, because it is 2 blu rays. I hate when they give the DVD anyway. I have a movie that is 3D blu ray, blu ray, DVD, digital copy, so you get 4 disks. I already own close to 100 blu rays not counting the 100s of DVDs. Space does become an issue.
For money, of course. No really, like Twilight (except for one, someone told me) - they want to milk the series, so if you want both, you have to buy both, but if you want one, then you have to choose.