Akira was, obviously, the launch off point for Anime in the west, and the movie was great. But later on I read the Manga, and they were so much richer than the movie. It seems like a 12-episode Anime would be perfect for Akira. And it would probably be successful, because everybody knows/loves it.
20th Century Boys is a bit... odder. I've read the Manga twice, and I know that there's a live action film, but the work seems so much better suited to Anime. I don't know how it's held up -- certainly no one on KIA2 mentions it. It's also not great -- the build up is fantastic, but the denouement is... odd.
Just wondering if there's a reason for this, or if it's just an historical quirk.
No one wanted to follow the Akira film. Its held up as a literal masterpiece and cornerstone of media, and anyone who attempted to re-adapt it would have to stand under that shadow. The only person willing to take that risk would be the type of slimey loser that couldn't ever measure up.
Look at Berserk. The 97 anime is hugely lauded, and because of that every re-adaptation has basically been mocked and laughed it even when they don't flop. And then people immediately go back to the original after the novelty runs out. Its a walking example of how a big shadow like that can easily ruin a work before it even releases.
As for Akira, it would also suffer with the fact that its main cast are basically hard coded into people's minds. Expanding on it would show different versions of those characters that would make people absolutely hate it. Tetsuo is far more sympathetic in the anime than he is in the manga, to a point where it would absolutely change how you view the movie in retrospect.