They didn't forget. The whole episode was pretty much setting that up from the moment Twilight encountered her future self. (SPOILER)Twilight was so focused on trying to prevent any "disaster" from happening, that when next Tuesday arrived nothing happened, she sent herself back to the past to warn herself, but was unable to before returning to present to continue on. Basically, you can watch this episode again knowing what happened and see that it's just history in a small cycle in this episode. It's a self-consistent and fixed history. Imagine if Twilight knew that "Future" Twilight was telling her there was no real disaster coming next Tuesday morning, this episode would be a second alternate timeline.
Okay, I've added a picture to help illustrate the timeline of the episode. I hope it explains much.
Edit: I put this post in spoiler since the episode is still new.
Edit 2: Added picture.
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