There are many different explanations as to why Equestria fell. The true answer is quite simple: Nothing can kill Friendship more than a mother having to bury her own child. Clockwork lived a long life, but Luna was blinded by grief.
This tale should be a warning to all to always live in the present, and not be blinded by goals for the future, or reminiscence about the past.
Edit: Featured 4/27/22