Because it could very easily have been over 80. Cardiff's defence has been shambolic this season, utterly awful - worse than Fulham's, in my opinion - and Marshall has kept them within even a shout of dignity. Of Cardiff's defence, only Caulker and Marshall can come out of this season satisfied with their performance. Juan Cala's played well when he's been on the pitch, but he's only had 5 apps. The real problem has been the entire team's transitions though: when they lose the ball, everyone sort of slowly jogs back into position. Compare that to Saint Tony's Crystal Palace, who lose the ball and immediately sprint back into a tight defensive block.
But I digress. Basically the reason why is because he's the one thing standing between Cardiff's defence and total embarrassment.