Why did Quora fail so bad?

IMHO, biggest reason it failed is due to SEO and marketers. Smaller reason for requiring Sign Up after seeing one answer.

Lots of people started abusing answers section advertising their products, they called it novel way of getting traffic at the beginning, then they called it everyone must have answers on Quora for questions related to their product, then they just started added bunch of non-sense just to link their product at the end or in the middle.

They also made difficult to read answers, making my sympathy towards platform decline day by day, whenever I need answer to something and want to give my attention to this website, they tell me: "Did you see first answer? Now fuck off from here"

I don't know if it's intentional, but I noticed a "fashion" on Quora that consists in attracting readers (and good old trolling) by making completely bogus claims in the form of questions.

So you'll see questions like "Why is the government hiding the fact that the Earth is both flat and hollow and that 27% of its crust is made of Swiss cheese ?"

The "response" sections of such questions have almost wasted enough of human time to qualify as a crime.

I think of reddit in the same way as quora. They are making it too difficult to access content. So I am starting to ignore search links that bring me to either site.

Fail in what way?