Teletoon's brand had actual value back then. In the late '00s and early '10s, shows produced for the channel were sold internationally to great success. Cartoon Network in the U.S. made plenty off of Bakugan, Johnny Test, Totally Spies, and Total Drama. It would've been silly to kill off a name that was launching hits just to fuel an American brand. Kids here would've been far more familiar with Teletoon's name than CN's at the time.
The problem is that Teletoon might as well be a different channel. Since Corus gained complete control of it in 2014, original content has largely vanished. Teletoon's schedule consists mainly of Warner's content, a few Canadian library series, and an even smaller amount of third-party acquisitions. Rebranding it to CN now isn't really changing much when the current Teletoon relies so heavily on that content anyway.
I've always suspected the Corus Cartoon Network and Nickelodeon channels might've just been a contractual obligation to keep access to those content libraries. The real money wasn't in operating those digital cable channels, but in recycling those shows on their far more widely distributed Canadian brand equivalents.
