That's a really good question, and it's crossed my mind here and there over the years. Obviously, a lot of Algo was somehow affected by all the bad things going on, from the expanding quicksand, to the illness on Dezo, to Mile, um, being completely obliterated by a magic hole in the ground. Eventually, those bad things got better (or at least that's what we assume), so a lot of the general public probably figured *something* happened. People already had their doubts about Zio, so finding out he was vanquished in some sort of good vs. evil battle probably wouldn't raise too many eyebrows.
If I had to speculate, though, I'd guess that some of the details weren't made known. One could argue that following the game, Rykros doesn't serve a purpose, so why get people all riled up with the news of "hey, there's been another planet this whole time!"? Same with the whole creation of Algo/Light vs. Darkness issues, especially since they don't really affect everyday life. Also, considering the schmaltzy end scenes where Chaz is sad about not seeing his friends again, I think it can also be safely assumed that communication between towns and planets wasn't that easy, and it would be difficult to spread a lot of information to a lot of people all at once.
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