Board Thread:General Discussion/@comment-31512992-20170313182842/@comment-27701762-20170313190803

There is a hint that the other humans did have the ability to save. If you tell Toriel your preference for butterscotch or cinnamon, then reset or reload and give the preference again (when she guesses which you prefer), she says that she has felt like she knew all of the humans like they were old friends.

As for why they died, that answer is not explicitly stated. There are only possible explanations. One explanation is that the humans encountered obstacles at which they finally gave up. Since saving is tied to determination, if you lose determination you should cease being able to reload. Another explanation is that the other humans had only a limited ability to save. It is commonly speculated that the special trait of the Red soul is determination, and if so then Frisk might be able to exert more control than any of the other humans.