Because at that time they figured cannon balls did as much destructive damage as possible yes they weren't accurate and cumbersome but fuck they did a shit load of damage when they hit targets properly even musket balls etc they may have been inferior to our modern day bullets but their aim was to maim and cause as much destruction and pain to human flesh so they still did the job I guess it was a simple but effective design and probably didn't take that long to produce or make them that's what I reckon anyway.