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The simplest answer would be because it doesn’t ordinarily work that way.
Well, Thomas Riker proves you can create duplicates and the doctor’s daughter in Strange New Worlds as well as some other episodes prove that the patterns can be stored in the buffer for extended periods of time.
Here’s the thing: Does Tom Riker actually prove that? That’s the explanation suggested in the episode, but the preponderance of information about the mechanisms of transporter technology, as given both before and after, conflicts with it. But there’s another hypothesis, a simpler one, and one that we know for a fact transporters are capable of, because it’s a recurring element in Star Trek: Thomas Riker is from another universe, brought to the Prime universe by similar means as many of the various visits to and from the Mirror universe.