I think it has to do with the *amount* of the leaves.

Exactly.

A lot of people think that a single layer is a fall's worth and they are
unfamiliar with leaves several inches deep in late november.

The other possible factor may have to do with the kind of grass that
the lawn has. If the lawn has warm season grass, the grass probably
turn brown or yellow when the weather gets cold in cold region. In
this case, someone probably can let shredded leaves on the lawn and
let them decompose. The color of the shredded leaves probably blend in
well with the color of the dormant grass. Actually, I don't know if
this is true or not because I have never seen this in practice.
Anyway, I have cold season grass on my lawn that stays green in fall
and winter. Shredded yellow/brown leaves simply cannot blend in well
with green grass.

Jay Chan