Steven Pinker explained this as a manifestation of the English grammar rule that headless compound nouns do not take irregular plurals. A headless compound noun is one that refers to an object that cannot be referred to by the last noun alone. For example, a mailman can also be referred to as a man, so it is not headless. But a Walkman cannot be referred to as a man, so it is headless and its plural is not Walkmen (for most people; I did a poll about it a couple of years ago). Likewise, when I talk about all the maple leaves in my yard, each one is not only a maple leaf but is also a leaf, so I can use the irregular plural. But when I talk about Toronto Maple Leafs, each one is a Maple Leaf but is not a leaf, so I use the regular plural.