Why were slaves from Africa preferred in North America over other types of forced labor (Native American slaves or European indentured servants)?

Explanation:

It was more profitable to have Native American slaves because African slaves had to be shipped and purchased, while native slaves could be captured and immediately taken to plantations; whites in the Northern colonies sometimes preferred Native American slaves, especially Native women and children, to Africans because ...

i believe the answer is:

-   william jennings bryan is a democrat while mckinley was a republican

at that time, the segment of population that choose for democratic party were the voters with low economic background and vice versa

- william jennings has support of populists and wants to have inflation to farmers while mckinley has support of bankers and businessmen , doesn't want inflation

the fixed value of gold make the large business able to stack up their gold without   being worried that it would devaluated.

- william jennings supported the silver standard while mckinley supported the gold standard

according to jenning, using other metal beside gold could the poor people to have more purchasing power and them advance in the society.