Answer:
To avoid the effect of hidden variables to affect the outcome of your experiment and keep consistency among all of them.
Step-by-step explanation:
For a scientific experiment to succeed you need to guarantee that there are no hidden variables that can affect the outcome of your experiment, this is why you what to set all of the five experiments as similar as possible, to reduce the effect of hidden variables and improve the likelihood of repeatability and future reproducibility.
The independent variable can not be held constant because it is the cause or reason for the outcome of your experiment; you can't predict how many flies are going to be in each pit trap, therefore, the independent variable changes.