Singapore has become the location for the US-North Korea summit for a number of reasons, including the level of confidence and experience that North Korea has in and with Singapore. North Korea has had an embassy there since 1975 and boasts comparatively strong ties with the country, at least compared to most others. One NGO in particular has been close with the North Korean regime, teaching business principles to thousands of North Koreans through in-country workshops as well as policy courses in Singapore. Connections like these have slowly built an important level of trust between the two countries, which has proven important in setting the location for the upcoming talks.
Source: The Singaporean NGO that shapes North Korean entrepreneurs | Asia Times