Answer:
Because a bigger difference in height would mean a larger difference in potential energy
Step-by-step explanation:
Water flowing from the reservoir to your tap is the result of the conversion of potential energy to kinetic energy. The bigger the change in potential energy, the more kinetic energy it would have coming out of your tap, aka faster flow. If we compare the height difference between the 1st floor and the reservoir to the height difference between the top floor to the reservoir, the latter would have a smaller difference, therefore smaller kinetic energy. So the flow on top floor would be less rapid.