Coincidentally, /u/DerProfessor touched on this today when answering the question What was the most hated colonial assignment in the empire for a British soldier? She states that, while quinine was isolated in 1820, it took several decades to become widely available to Europeans in Africa.

But I'm very wary of Headrick's argument about malaria, though. In an error all-too typical of historians of technology, he tends to emphasize the dates that the technology (in this case quinine) was first discovered or introduced, rather than the dates that its usage became actually widespread enough to be significant. When you get to when quinine was a standard prophylaxis for Europeans stationed in West Africa, we're already well into the new imperialism of the 1880s... (as the dates in Headrick's own footnotes show). So Headrick's argument that this was a precondition for conquest just always seemed off to me.

That whole thread has some interesting information on Quinine. I notice /u/TruePoverty chiming in quite a bit on that topic.

It seems like there's room to expand on why it took so long for quinine to become widely available, but hopefully that thread provides you with some good information.