Madara established that no one could effect him; because when he stabbed Sasuke, he said that he'd do anything to complete his goal--or something-even if it meant killing a brother (Izuna, whom Hashirama compared Sasuke to). Hashirama made the point that because Sasuke was similar to Izuna that he could change Madara, but clearly this was proved incorrect since Madara said he'd even kill a brother. Thus, this proves Hashirama's theory wrong. I feel like Sasuke was stabbed so easily because when he approached Madara, it was an attempt to make peace. I highly doubt he was just that weak.