The real title should be: Why does math sometimes seem hard to learn?
Researchers think it has to do with the abstract nature of most concepts in mathematics. In other words, it can be difficult to make a genuine connection to a new idea in math, like "what is a logarithm." On the other hand, it's easy to understand something like like "ice on the sidewalk is slippery" or "that thing in your hand is a shoehorn."
Other abstract concepts not related to math are much easier to grasp: they are often related to emotions and therefore different because we have personal experience with them at every age. The good news is that anyone can master difficult new concepts and become good at math by following a few simple approaches to studying and learning.