Because the hanging mass has to be exactly the same as the mass of the spinning object; without the point masses, we need to reduce the mass on the hanger.

The bar has a higher rotational inertia without the point masses, so we need to use a smaller mass in order for the acceleration to not be too large.

So that we have two data points for comparison - one with 50g hanging mass, one with 10g hanging mass.

The bar has a lower rotational inertia without the point masses, so we need to use a smaller mass in order for the acceleration to not be too large.