Why do scientists change only one variable in a controlled experiment?
To look at the effect of changes in that variable on the outcome of the experiment.
If more than one variable is changed in an experiment, scientist cannot attribute the changes or differences in the results to one cause. By looking at and changing one variable at a time, the results can be directly attributed to the independent variable. Thus coming to the conclusion of the relationship between the variable and the results, whether the relationship is a correlation or causation.