The main reason that it's important to test only one variable at a time is so that

This is to make sure the test is reliable because it can happen that the two variables being tested affect each other skewing results

Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.