Because, when LL first broke the news that it was going to be switching over to a brand new SL, eventually leaving this one behind, most land owners said, welp, time to pack it up and leave.
Then you had people saying to stick around and support LL, because if a bunch of people left, LL would re-evaluate their stance and say that the current SL version should be trashed with no warning. My response, what's the point in staying at a place where there is zero activity? You get bored quick, you see all the other people cutting their losses and just up and leaving.
If I had to do it all over again, I don't think I would have gone back to SL. A friend told me about it, I joined, a few months later he leaves, I couldn't figure it out so I quit, and then came back to it some time later on, and would stay with SL for a total of about 4 to 5 years.
It's an extremely expensive world to be involved in, and that is why most of the people are leaving. I've left SL without looking back, but I have no intentions of ever returning.