Lets assume that you want to store ItemList<T> in an array.
Allocating an array of value types (struct) will store the data inside the array. If on the other hand ItemList<T> was a reference type (class) only references to ItemList<T> objects would be stored inside the array. The actualy ItemList<T> objects would be allocated on the heap. An extra level of indirection is required to reach an ItemList<T> instance and as it simply is a an array combined with a length it is more efficient to use a value type.
After the inspecting the code for EntitySet<T> I can see that no array is involved. However, an EntitySet<T> still contains two ItemList<T> instances. As ItemList<T> is a struct the storage for these instances are allocated inside the EntitySet<T> object. If a class was used instead the EntitySet<T> would have contained references pointing to EntitySet<T> objects allocated separately.
The performance difference between using one or the other may not be noticable in most cases but perhaps the developer decided that he wanted to treat the array and the tightly coupled count as a single value simply because it seemed like the best thing to do.