Tor: Why do some websites block Tor users?

Short answer

Websites see shared exit-relay IP addresses and may apply automated abuse controls or explicit policies to those addresses. This can block legitimate visitors alongside unwanted traffic. Try the site's support channel, a different circuit, or an official onion service if one exists. A block is the site's decision; Tor Browser cannot promise access to every website.

Sources

Tor Project: Website blocking, Tor Project: CAPTCHAs.