sp_cursoropen and parallelism
FAST_FORWARD cursors do not support parallelism (though the server generating the plan would need to be 2012 or above to get NonParallelPlanReason as part of the showplan XML).

When you specify FAST_FORWARD, the optimizer chooses between STATIC and DYNAMIC for you.

The provided execution plan shows the optimizer choosing a static-like plan.
Because the query contains aggregation, I doubt a dynamic cursor plan is even possible here. Nevertheless, requesting a FAST_FORWARD cursor type is preventing a parallel plan.

You should change the cursor type explicitly to either STATIC or KEYSET, for example. Both these cursor types can use parallelism.

That said, because this is an API cursor, changing the type of cursor would likely require an application change. Naturally, you would need to benchmark performance to check that changing the cursor type really is the best option for you.