It's the same reason that your computer now needs 8 gig of RAM to work properly, when a few years ago you could get by with 256 meg.
I didn't realize this was coming. That's great news for me, as I'm currently on a grandfathered 18gb plan for our 4 phones, which costs me $20/line plus $100 for the data plan, but I get a $20 company discount on that. So it totals $160, and the current unlimited plan is $180 with no company discount available. So I'd welcome the ability to pay the same amount (as I'm paying now) and go to unlimited, even if it has some restrictions.
I look at things in a somewhat less cynical way. They previously offered unlimited with 3G service. As the world moved forward to 4G, it was like starting over.I have to look into my plan again. VZW screwed me out of my unlimited plan when I got my Note 4 3 years ago.
Now the Unlimited's are back. I'm sure they are going to take something else away from me again.