Is this IRL or something? Because I don't see any of this Arthur hating you keep talking about in any public areas on the internet, like forums and comment sections and such.
If it is real life, it's probably the age-old, "That's for babies!" jive that people always use as an excuse for saying you're not supposed to watch stuff like that... I know, by the time I was in Grade 3, Muppets of any kind were considered "for babies", and by the time I was in Grade 8, suddenly, watching cartoons was "uncool" and showed that you "needed to grow up" (but like you're complaint also regards, at that time, SpongeBob was considered the only exception).