Well, if you want an in universe explanation, it could be said that Jedi of the Master rank didn't handle any peace keeping duties until the Clone War. Peace keeping was left to Knights. Once shit hit the fan, then the Masters got involved cause their numbers were dwindling so much. So what Obi Wan said was true, from a certain point of view.
^Err, no, because I'm talking about the actual use of the word "Knight" in the films and TV series. I'm not a follower of all the Expanded Universe whatsit. I'm just a casual viewer. I've seen the movies and I've seen the Clone Wars TV shows (plus I saw the Holiday Special back when it first aired and the Ewok TV movies and a bit of the early cartoons). And I read the early books like Splinter of the Mind's Eye and the Brian Daley Han Solo novels and a few more since. And in my experience as a casual viewer, it seems to me that I'm hearing "Jedi Knight" less often than I used to. I'm not even sure if that's true, it's just my impression. I'm not asking for some in-universe handwave, I'm just asking if my perception can be verified that in real life the people who write the scripts for Star Wars productions are less inclined to use the phrase "Jedi Knight" than they were a few decades ago.
's reply is actually spot on.
His reply addressed what I was talking about, not what you were.