Because between 15 and 20 years ago the BSD zealots won that culture war. I would really love to see a simulation where GNU licenses won that culture war and which landscape we would have today.
GPL wouldn't have changed anything. Or even AGPL, if you had been running an unmodified Bear instance.
There are good reasons the so-called "BSD zealots" won the so-called "culture war", such as the definition of a derived work being largely uncharted territory. Did you know that there's a compelling legal argument that the GPL attaches to programs talking to each other via JSON data structures?
I think MPL 2.0 is the ideal kind of copyleft, because of its scope being very clear.