There are different speculations as to why Nikki Haley resigned. There are some who believe that she has resigned because she would like to run as the president of the United States in the year 2020. There are also some who believe that she is just one of those people who first believed in Trump but saw later on that there is nothing to believe in.
President Trump tried to downplay this resignation by stating that this was planned months before. It should be remembered that Haley is a politician. People can only speculate but it is very likely that her resignation from Trump’s cabinet is politics-related.