Why did General Sheridan choose to attack Native American tribes in the winter?

Tribes were most vulnerable in freezing temperatures.

Because in 1867, Ulysses S. Grant charged Sheridan with pacifying the Great Plains, where warfare with Native Americans was wreaking havoc. In an effort to force the Plains people onto reservations, Sheridan used the same tactics he used in the Shenandoah Valley: he attacked several tribes in their winter quarters, and he promoted the widespread slaughter of American bison, their primary source of food.