What factors led to the demise of yahoo answers besides declining number of users?

Category shift abuse was also a very big factor why many stopped using it. 

When you name your website rude noisy violent. It dooms you to failing inevitably! Wicked people seem to get the upper hand for a while and they get a chance to rejoice and gloat over the people that they stomp on.

But in the end the truth meets up with you. 

People have left for many reasons in recent years. Some of them are:

Lack of oversight of YA by Yahoo shortly before the sale to Verizon

Lack of oversight by Verizon after they bought Yahoo

Chronic glitches and issues with the site that have never been fixed

Influx of trolls and site abusers, going unchecked, due to no YA Team

Massive spambot attacks in 2018

The temporary addition of a captcha feature (to weed out spambots) annoyed some users enough to cause them to leave YA

imo only.....It is a private business who has made a decision after much deliberation I am sure.

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 New laws are coming out that will hold social media sites responsible ( libel) to clean up what is posted ( racism. slander, abuse ,porn etc...FAST or face fines = $$$$$$$$$$$