It's a nostalgia grab.
"Hey, remember when you had fun playing this? Do you remember?"
Of course, cash goes right along with that, but that's the primary thing they're going for. They're just aiming it at nostalgia, so that people won't think it's a cash grab.
*shrug* I didn't really love D2 anyway. Loved what D1 *could* have turned into. They could have turned D1 into a sharp tool against evil in the world. They *could* have made D2 with specialized quests for each character, wherein they each faced down their greatest fear.
Then when I played D2, I was just like, "What the fuck is this bullshit...?" Just silly magic and crystals, and all that bullshit. I agree, their mechanics were some of the best out there at the time. But mechanics (and magic, just because it's magic) just aren't enough to keep me coming back.