I think Kevin's mistake was to keep Miranda out of the fight for so long. The conner net drop was neat but in the end, ineffective. Nand flew very well. Both players deserved to be there. I watched an earlier game of Kevin's and he flies his list very well. Good strategy and use of both ships. I think he was nervous and tired in the final game. Once Nand killed Corran it seemed to take the wind out of Kevin's sails. The chance that Miranda could win against these 2 ships alone was astronomically slim.
I believe he conceded when he realized he didn't have a solid strategy to win anymore. Sure he might have taken one of Nand's ships down but at the end of the day it wouldn't have made a difference. The game was over once Corran was dead.