Why cant I use "...." at the end of the sentence?

I would imagine that the use of an ellipsis would not score you any points with a college admissions officer.  There are several reasons that one uses this form of punctuation.  Basically, it means that something is missing. 

If you are quoting something, an ellipsis would be used when part of the quote is deliberately deleted.  This is in the middle of a sentence & used to shorten a quote.

When used at the end of a sentence, it indicates an unfinished thought.  It asks the reader to finish the thought for himself.  A college admissions officer is likely going to want to see that you can complete a thought yourself. 

When writing about people talking, such as a script, the ellipsis can be used to indicate trailing off into silence.  If you are submitting a creative writing sample with your college application, then this is the only place in your application that it might be used.