What made Cartoon Network special was that it was a specialized channel FOR cartoons. I believe in the 70s and 80s, cartoons would air on curated blocks on ABC and CBS, hence the Saturday morning cartoon tradition. I think Nickelodeon did not start airing cartoons until the 90s either. That was unheard of for the time.
Ted Turner had acquired rights for several Looney Tunes era cartoons in the late 80s. Animation had begun to have a resurgence thanks to airing Hanna Barbara cartoons. Ted's philosophy was to have a station geared towards animation to take advantage of changing trends. Strike the iron while it's hot. It paid off tremendously.
Even more interesting, those cartoons that would rerun on Boomerang started as ABC/CBS/NBC programming. That license acquisition was Hanna-Barbera, which had lots of animation suites under their belt.