This is a much more extreme version of the same reason I never used Quora. I'd be allowed on one page, but after that, clicking on any internal link in the site would push a "create an account" dialogue and wouldn't let me click off to see the content. The way around that was just reloading the page, but the irritation and tiny extra step was enough to stop me from browsing that site. Just checked right now and Quora doesn't do this on PC, but it does on my phone.

Now Twitter won't let you on the site at all without logging in.