Question

why is cotton picked over a long period of time

Answer 1

 Cotton has to be picked in a specific period of time, otherwise lint drops off from the boll and the decreases the strength of the cotton.

Answer 2

Traditionally, cotton fields had to be plucked by hands usually three or four times in every harvest season. The bolls on the plants could issue at different rates. Producers could not utterly neglect the early-maturing bolls on the plant till all had culminated because the property of the cotton worsened as soon as the bolls developed.