Why is cholera still a problem for many countries, even though people learned much about the disease and how it spreads during the london epidemic of 1846?

Because many people don't get clean and safe drinking water

Simply because many communities do not have the resources or infrastructure, i.e. no wells, sewage systems etc. Instead, they are forced to drink water contaminated with bacteria. 

Also, many do not have an understanding of how the disease is transmitted and the ways in which it can be prevented, such as hand washing.

NGOs, such as WaterAid, have tried to limit the spread by building wells (using local labour) and educating communities on how to maintain them. Aid workers have also introduced soap and taught children how to wash their hand properly.