Because you have total control over it.
In any experiment, the independent variable is the variable which you can control and manipulate.
Various values given to the independent variable will cause direct variations in the dependent variable. In fact, this is what experiments are all about - you get to test the impact changes made in certain parameters, i.e. the independent variables, has on other parameters, i.e. the dependent variable.
In your case, the mass of zinc powder is considered an independent variable because you control how much zinc can actually take part in the reaction.
By comparison, the amount of copper produced by the reaction will be the dependent variable because it depends on how much zinc you use.
For example, another independent variable you could chose is pressure. You can have direct control over the pressure at which a reaction takes place by using a pressure chamber .
That way, you can determine how pressure affects a chemical reaction.