Sanctions can starve people already of borderline health such that it was seen as heartless by the world before. USA could have been charged with genocide by the UN and others. Plus, China doesn't want refugees flooding in.
But Kim's recent missile and nuclear successes and threats change the sanction pressure and realities enough that Kim realizes his country is now more likely to be starved to death if he stays on the same path. The collapsed testing site also highlights the problems and resources necessary to maintain such systems.
Therefore, he's more open to negotiations than before. Kim built a better bargaining chip but also opened his country up to sanction-induced mass starvation.
Note that initial goodwill has fizzled multiple times before with the ruling family, so it's premature to say what the results will be.
