It is "easier" to explain things when there is only one of them or many of them, but not in between. For instance, once we found there are many tens of elements

It is "easier" to explain things when there is only one of them or many of them, but not in between. For instance, once we found there are many tens of elements, it was easier to group them in a periodic table, leading us to the discovery of just two (three) elementary particles that explain all elements. Once we found there are many tens of elementary particles, we were able to classify them in tables that made it obvious we need only a handful of leptons and boson to explain them all. The vague hope is that finding a new force will make it obvious what is common between all of them, leading to finding the simpler underlying object that explains them all.