I think there are many factors as to why the Europeans depended more on enslaved Africans, one you mentioned is epidemics.
When the Europeans brought over disease there was a decrease in the Native American population, on top of that indentured servants were more expensive than African slaves. So when they made Africans Slaves it cost less, and unlike the Native Americans they didn't know the area that well, so they weren't as likely to escape.
I hope that helped answer your question!
