Why is it bad that a scientific test is only done once?

Not "bad", just not good. Repeatability is one of the criteria for establishing a thing as true rather than something due to happenstance (random conditions that are not recognized as important).

Doing it once says it is possible. Doing it many times says that you are in control and understand what matters.

Lots of things get suggested from a one-off result. The only way to be sure about what actually matters is to try it many times, changing things a little bit each time to see what effects they have on the result.

Doing an experiment once is not bad, because you still learn from it. You simply do not learn everything you probably need to know from the one try. You need to do more to be sure what is happening.

Because it's possible that the result doesn't represent what would happen if the test were repeated.

Yes