For situations where Tor is blocked by a firewall, you can configure Tor Browser to use ShadowsocksR (SSR) as a proxy server. This short proof-of-concept video demonstrates the client configuration on Windows.
Tor浏览器+ShadowsocksR(SSR)客户端配置 Client Configuration
This can have several reasons:
Tor Browser comes with HTTPS Everywhere. This addon has rules for several sites to connect via HTTPS. At https://atlas.eff.org/index.html?tubes you can see all sites which contain tubes. Your site is not among them.
There is a chance that the site itself redirects your request to the HTTPS. You can check it with curl. Just ...