Because Tor is the onion routing network.
When we were starting the new next-generation design and implementation of onion routing in 2001-2002, we would tell people we were working on onion routing, and they would say "Neat. Which one?"
Even if onion routing has become a standard household term, Tor was born out of the actual onion routing project run by the Naval Research Lab.
(It's also got a fine translation from German and Turkish.)
Note: even though it originally came from an acronym, Tor is not spelled "TOR".
Only the first letter is capitalized.
In fact, we can usually spot people who haven't read any of our website (and have instead learned everything they know about Tor from news articles) by the fact that they spell it wrong.