Why might there be water scarcity in the future

Because there's only a certain amount of freshwater that we can drink and in the future that certain percent is going to be lower. 

I’m not sure if you are looking for an exact answer for a test, but there are many reasons for this.
As humans continue to consume (fresh) water, it starts to run out. Of course, we release this water through our bodily processes, but not all of this returns to be used again. Whenever water evaporates, it usually turns into clouds (I’m sure you already know this lol) but when it rains, not all water returns to fresh water lakes. A great amount either directly falls or trickles into the ocean, contaminating it for drinking.

Another reason could be access to water. Even if water is abundant, it can be very hard to access. Most freshwater is landlocked, either in bodies of water, like lakes, or trapped beneath the ground, called “fossil water”. Places either on the coast where they only have salt water, or very dry places, can struggle to get the water they need.

I personally don’t think we will ever have a water crisis, but there is the possibility if we don’t monitor and conserve it well.

Ps: I just realized I basically wrote an essay for this lol. Hope this helps

I personally do not think that we will ever have