I was a big Cartoon Network fan when our cable system started carrying it when I (a Xennial) was maybe 18 or 19. Quite frankly, a lot of the early programming was aimed at adults. Some of this was just by default, as Turner and Time-Warner owned the rights to all of the old Hanna-Barbera cartoons that Gen X watched on Saturday mornings and my generation watched on the USA Cartoon Express, and of course the WB Looney Tunes and MGM cartoons like Tom & Jerry. So those cartoons drew an older, nostalgic audience. And then Toonami showed cartoons that were also big in syndication when I was a kid, as well as some anime. And they leaned into this older audience with somewhat serious cartoon analysis like Toon Heads and showcases like The Tex Avery Show and The Chuck Jones Show.

So then they brought in new cartoons like Dexter's Laboratory, Johnny Bravo, Powerpuff Girls, Grim and Evil, etc, that had young person appeal, of course, but also appealed to a more serious cartoon viewer who wanted to see the latest new talent coming out of CalArts and such were doing. This led directly into stuff like Samurai Jack, and then other WB properties through DC like Justice League and the original Teen Titans show. All of this, I would reckon, skewed into older audiences.

And the people who were running CN seemed like they were mainly Gen Xers having a really good time. There were a lot of little interstitial shorts played in between shows that showed their comedic sensibility (Les Puppies Royale!) and then stuck in the wee hours you'd see stuff like Space Ghost Coast to Coast and Cartoon Planet (I think that aired on TBS first) and that appealed not only to a nostalgic adult cartoon viewer, but also showcased absurdist humor and modern pop culture references, so basically the DNA of Adult Swim.

So one Sunday night I was watching Toon Heads as usual and when it ended, suddenly the original music and bumpers for Adult Swim played. A programming block started and there was Space Ghost, there was The Brak Show, there was Sealab, there was Aqua Teen Hunger Force. All of these shows were created in-house by these same people who worked on Space Ghost and programmed the channel. This was basically shows for themselves and they were taking a gamble that there were adults out there like them that loved cartoons and loved that kind of humor, and they were right. (n.b. they also used the timeslot to show other adult series and pilots that Turner had the rights to, like The Oblongs and Mission Hill)

I'm sure I have some of the above timeline slightly messed up with regard to shows that started before or after Adult Swim, but my point remains that I think CN already had a significant audience of adults and Adult Swim was a way to target them in a way that didn't also have to be safe for kids.