Actually, its a matter of manufacturability/cost and preference. The handy boxes were used for many years prior to the welded box. The handy box being drawn, requires a bit thicker and more ductile steel to form properly, and has a few more steps to make it (2-3 drawing steps, punching KO's, and tapping holes), whereas the welded box has three components all stamped and tapped in one shot in a progressive die press, then they are resistance welded.
Plus the handy boxes are mainly surface mounted, and the welded boxes w/brackets allow stud mounting and easier depth setting capability..