It would make sense that something so profound would have multiple sources confirming that it happened, but all we really have to go by is the word in the bible.
Disclaimer: I'm not trying to offend or prove any religion wrong, I'm just curious as to why there aren't multiple sources.
There are some problematic assumptions in your question.
If by "scientific evidence" you mean "proof that conforms to the scientific method which assumes methodological naturalism", and by "miracle" you mean "event which is caused by a non-natural cause", there can never be any scientific proof of a miracle, because science has by its own method ruled out the possibility of proving a miracle. This is why science can only ever disprove miracles by showing that such events had a natural cause, and the best science could ever do would be to state that it had yet to find a natural cause for any particular event.
The Bible is not a single document, it is a collection of sources.
Given the nature of sources from the 1st century, I would not expect multiple sources confirming any particular event, miraculous or otherwise, to necessarily be available. I'm always more surprised at the sources we have than at the sources we don't.