Q: Why is it difficult to play between the black piano keys?

Piano keys are a fulcrum and as we all know from being children, fulcrums are lightest on the edge. Sit on a see saw and notice if you sit inward, you weigh less and the opposite person has the advantage. Sit on the very outside edge and you weigh more.

So, playing on the outside of the piano key gives you more power and the keys are easier to depress because they require less force. If you play on the inside of the key, in the black area or, closer to the fulcrum, the keys will be harder to depress.
However, this is only predominately true if you have poor technique. There are several ways to give the arm more power to the fingers such as proper alignment, rotation, in and out motions because your fingers are different lengths and a fulcrum unto themselves, gravity and basic laws of physics such as every motion must have an equal and opposite motion.  Then, there are things not to do which if we do, will weaken or diminish our virtuosity. 
Some teachers just teach dot matching and don’t have a knowledge of physics, biology and ergonomics. If your teacher doesn’t know what a pronator is or how to adjust the elbow so the four and five fingers are just as strong as the other fingers . . . that teacher may hurt you.  If you are lucky, you will only be mediocre.