why is yahoo shutting down yahoo answers ?

I think it's because they don't want to deal with all the political controversy. We have to wonder if it's a crack down on free speech. If Yahoo needs to make a profit, they could always have an ad pop-up when you first log on, and then goes away. 

Answers is a Non revenue producing entity. If they want to make it viable

for a company. Charge a Nominal fee for the usage of the those who wish

to obtain the service.

It's not making a profit, and it's infested with trolls. Without profits, there's been no point in wasting time, money, and staff on policing the trolls. Better to get rid of them by shutting the whole thing down.

I think it's a smart decision.

Same reason they closed comments.

To control what you see and think.

To force you to believe false information.

Propaganda.

Verizon wireless owns it now. Show your displeasure by switching phone companies.

These decisions usually come down to a matter or revenue.  Yahoo Answers is probably not making enough money for Verizon anymore, and the determination has been made that it is beyond worth fixing.

Why Y!A has reached this state though, I think can be traced by the last 8 years of site mismanagement on the part of Yahoo and Verizon.  They made some bad changes to it in 2014 and seemed to start treating it as a liability around 2018.  Frankly, I'm surprised it wasn't shut down then.