To save weight. All that top-hamper weighs quite a lot and it's mounted very high up. Cutting it away reduces topweight significantly.
Makes sense. Thanks. There aren't many pictures of the Atlanta with the new masts, and only a few more of the Boston, probably because the Atlanta was decommissioned in 1912. The Boston hung until 1946 first as a training ship for the Oregon Naval Militia (1911-16) then as a receiving ship at Mare Island from 1918 until 1946. She was renamed USS Despatch in 1940 so the new Baltimore class cruiser Boston (CA-69) could have the name. In 1941 Despatch received the hull number IX-2, which as far as I can tell, was the only hull number she ever had, never being assigned one during her years as a cruiser. Last I heard, the Boston's pair of main 8 inchers are in a park near Seattle.