C-46s carried much more payload than the C-47 but could not operate out of some of the more primitive Pacilfic fields. Curtiss had significant problems fulfilling it's contracts for other designs, to the point that government procurement people got involved and in some cases took over Curtiss' plant operations. That is likely the biggest reason why more C-46s weren't built. Another reason was the stiff competition for it's R-2800 engines for P-47s, B-26s and F6Fs. Douglas, on the other hand, had long and well established production lines dedicated to producing DC-2s and then DC-3s well before the push for wartime production.