Same reason that your favorite footballer breaks in Premiership. Poor diet and lack of sleep form the foundation for a weakening immune system, which can be especially relevant if they play on rain soaked pitches.
Drinking to excess causes the body to lose hydration and electrolytes so it can't function correctly. Dehydration is one of the most common sports injury related-issues because during exercise there are higher sweat rates, shorter times between drinking opportunities, more people are relying on fluids instead of food for energy sources....while also cancelling out potential weight loss with dehydration calories consumed from booze will actually worsen their condition.