This and also: NYC residential trash collection is curbside. Literally leave trash bags on the curb. Trash collection cannot occur when the curb is snowed in.

So, were there separate trucks for the two tasks, the trash trucks would sit idle until after the snow plow trucks had finished their rounds.

Given the infrequency and relatively small volume of snow NYC gets, plus the curbside trash collection, sharing trucks and drivers is reasonably efficient.