A lot of reasons: the show has been on longer than Astro Boy; if you're talking about the 2003 series. However, even though Astro Boy has been a big hit since the manga and the 1960 and 1980 series, it hasn't gathered much of a crowd since then.
Ever since Spongebob has been on, which was back in 2001, the show has kept a large crowd, plus it kept continuing the series by making new episodes.
Sure, Astro Boy had three seasons with plenty of episodes to remember by, though today we have been looking for new stretches of comedy to satisfy the fanatics.
At least Cartoon Network and Boomerang decided to show its 1960s and 1980s reruns; otherwise I'd go straight to torrents.
