Though in Curious George's case, he already had a successful book series going for many decades to help fuel that.
Mickey Mouse is pretty much the most universally recognized character out there and he still has to stoop to being a Dora clone. Sure, George did get the push of the movie version (which the cartoon is clearly based off of) but they didn't need to dumb anything down when they made the series. They even kept some very sly humor into the series (the Man with the Yellow Hat obsessing over his hat, commenting once that if he lost it, he wouldn't know who he'd be). And while I never thought of Arthur as a preschool show (Kindergarten through second grade maybe), the show has a Simpsons level of satire to it, even making episodes that were satires/parallels of Shakespeare works. My sister had to point out one of the episodes was supposed to be King Lear.
I find it complete and utter rubbish that kids will only learn or be entertained by something that answers their own questions after 20 seconds (Dora, Mickey Mouse Clubhouse, and worst of all Super Why). This is an unfortunate trend in preschool television that I can't wait for to end.