I think there are several advantages of stack allocation:

* freeing stack allocated memory is O(1) with a small constant factor: simply set the stack pointer to a new location. In a generational garbage collector, like OCaml, minor garbage collection is O(amount of retained memory) with a larger constant factor.

* judiciously stack allocating memory can improve data locality.

* unboxed data takes up less space, again improving locality.

Overall, I think this about improving constant factors---which makes a big difference in practice!