In a letter to Kim Jong-un, released to the media, Trump accused North Korea of hostile statements which, according to him, make the summit impossible:
Sadly, based on the tremendous anger and open hostility displayed in your most recent statement, I fell it is inappropriate, at this time, to have this long planned meeting.
Since the very first summit talk National Security Advisor John Bolton set impossibly high expectations for the results. Trump fell for it.
The various 'hostile statements' go back to remarks by Bolton who has for some time compared disarmament of North Korea to Libya. On April 29 Bolton again asserted that the 'complete de-nuclearization' of North Korea would follow the 'Libya model'. North Korea never really offered to 'de-nuclearize'. It rejects the 'Libya model' for two reasons: