It's because they use JavaScript. Even if you visit the clearnet link and disable JavaScript from your browser settings, you will get a blank page (Your Tor settings are probably set to disable JS).
So that means they are going against their own reccomendations on their "Step-by-step guide on how to mix your coins" article?
Assuming these coins are going to be sent to a darknet market… if you don’t already have your deposit address, sign in and get it while having JavaScript disabled. Never use any market that requires you to enable JS!