To my most faithful student, Twilight Sparkle,
I see from your letter that the rash of disappearances of pegasus ponies has not escaped your notice. I find that I should inform you there is less publicly available information that causes this recent turn of events to remind me of a similar event 300 years ago.
Those events were resolved by a pony named Lightning Dash. Remembering that fact caused me to do some research and as you might have guessed this Lightning Dash was in fact one of Rainbow Dash’s ancestors.
Unfortunately, at the conclusion of said events Lightning Dash never told me where he had found the missing pegasi. Out of respect for his decision those rescued chose to remain quiet on the mater as well, so I can not tell you where I suspect Rainbow Dash is.
Please believe me when I say that I have been around long enough to notice that the patterns of history often repeat.
For these reasons I find Rainbow Dash’s disappearance both disturbing and a great relief. As I suspect Rainbow Dash herself will be the key to solving this rash of disappearances.
You no doubt wish to aid your friend. However, with no practical means of doing so, I can only advise that you pray for Rainbow Dash’s wellbeing and safe return.
Finally, the attached document is a copy of my public declaration regarding the matter of the missing pony cases. Please forward it to Ponyville’s mayor as soon as possible.
-Celestia
Thus Lightning Dash's greatest error. NO TELLING ANY PONY! So if it happened again the ponies will be unable to prepare or assist..
7865023
I might not always agree with you but when I do it's with great gusto! Especially in this case cause unless a good reason can be provided for him not doing so Lightning Dash's choice not to tell is practically criminal in several major ways.
8257925
The short version would be:
Knowledge of the unknown breeds curiosity, curiosity breeds experimentation.
That and someone understood that there would always be unicorns like starlight glimmer around (reckless, powerful and of questionable ethics,) and that the easiest way to keep the words from interfering with each other was to not tempt them to figure out how to make portals to the other word.