You people who have this obsession over Atlantica seriously need to get over it.
Before we knew BbS existed (hell, before KH2 was announced), how did you think Triton knew about the keyblade? Gee, maybe he just, you know, picked up on it like any legend or found out by some other non-relevant means. Why can't you leave it like that?
The point is that it shouldn't matter how he found out. Had they included Atlantica for some odd reason, great, but Triton's knowledge of the keyblade is certaintly not the grounds they would use for including it. Completely unnecessary. Just because BbS is a prequel does not mean that every minute detail of previous games has to be explained.
Really though, as has been said, there's just not enough story they should flesh out for Atlantica, and that ought to be the reason why it didn't make the cut. Sure, Little Mermaid 3 provides backstory so that they could include it (and aknowledge the time shift with a younger Ariel and Triton), but that doesn't mean it should. Because LM3 blew.