Well, Curtis was a manufacture kinda on its way out. It spent alot of effort churning out P-40s and Helldivers and I imagine that ate up alot of its production capacity. Additionally, the fabulous success of the C-47 probably had something to do with it.
Well America did like to have redundancy . . . B-17 and B-24, B-25 and B-26, P-38 and P-47 and P-51. I know they all managed to find their own little roles but they all did the same basic mission.
The sources I've been able to check show the C-47 cruised about 2mph faster which isn't much until you consider that the C-46's top speed was 40mph faster.