Why doesn't yahoo just leave the site unattended and alone, no moderators, instead of shut it down?

Because it's a burden to them. 

Because there might be a law change where social sites are responsible for the content posted on their sites. A fully unmoderated YA would destroy Verizon.  They are getting out while the getting is good  -to use my grandfather's turn of phrase.

It's likely an legal issue...which is also why comments were removed.

You haven't been paying attention.

Moderators are long gone, it's been an unattended cesspool of spammers and insults for years

That's what they've been doing for the past 7 years. I guess there were lawsuits or something.