The purpose of the cause-and-effect structure of this passage is to mark important events in the history of sugar and show how the desire for sugar led to slavery.
Firstly, if refers to the an important event in the history of sugar: when Europeans bought is as a replacement for honey. Since people liked sugar better than honey and it was much more cheaper it became greatly produced and bought.
Because of the large demand for sugar, Europeans resorted to slavery to carry out all the new and heavy tasks. Therefore, the introduction of the new sweet substance led to the most brutal of labours.
