Curious about something. Would a game budget be different if you are shooting for 30fps compared to 60fps? If so perhaps that's why? If they don't feel something needs to be 60fps why put in the extra money and man power when 30fps is good enough for what is being shown? Could that be the thinking taking place?
I think its the resolution/other graphical effects they are aiming for alters the budget, since the more they go into HD they need to produce assets that match that.
Seems to me it's more about development cost than technical approach (with NSMBU at least). They have talked a lot in the past about the cost of HD development and how they were trying to lower it.
Its Nintendo. Are they really strapped for cash that they have to make a budget Mario game? Its probably the same engine from the first New Super Mario Bros so the engine is sorted, so how much would it really cost for that?