Mainly because of how intense he is on set, as seen in this excerpt from wikipedia:
"Day-Lewis put his personal version of "method acting" into full use in 1989 with his performance as Christy Brown in Jim Sheridan's My Left Foot which won him numerous awards, including the Academy Award for Best Actor. During filming, his eccentricities came to the fore, due to his refusal to break character. Playing a severely paralyzed character on screen, off screen Day-Lewis had to be wheeled around the set in his wheelchair, and crew members would curse at having to lift him over camera and lighting wires, all so that he might gain insight into all aspects of Brown's life, including the embarrassments. He broke two ribs during filming from assuming a hunched-over position in his wheelchair for so many weeks."
I hope he sets the Best Lead Actor record with another win.