The venue had to be a neutral venue where neither side would have "home ground" advantage. Mongolia was the other candidate. Unfortunately, Singapore was picked. The "honour" of hosting a summit between POTUS and Supreme Leader was too much for Singapore to decline.
(Yea, we don't want them here. All those security would just mean road closures, delays, and potential threats and security screenings. This all means "inconvenience" to Singaporeans, so "thank you, but no, thank you". Buuuut. It's apparently a done deal. Or is it?)
Then US and South Korea carried out their annual military exercise codenamed "Max Thunder".
Or Trump and the US were braying triumphantly about how they have succeeded in bringing North Korea to the denuclearisation table, and that pissed off the North Koreans (actually, it pissed off the ONLY North Korean to matter - Supreme Leader).
So they have been making noises threatening to call off the talks.
Well, Singapore is ranked one of the most expensive cities in the world...
Kim: What's this?
Aide: That's the invoice from Singapore.
Kim: Invoice? For WHAT?
Aide: For our meeting with Trump. In Singapore. On 12 June.
Kim: What are we supposed to do with it?
Aide: I think they expect us to pay it.
Kim: WHAT???? They think we made of money or what? $10,000 for the Presidential Suite? Per night?!?!
Aide: it's the Shangri-La, Supreme Leader.
Kim: Send the invoice to the US. Tell them to pay it!
Aide: Errr... it was the US who sent the invoice.
Kim: What?!? They expect us to pay? Such insolence! Cancel the meeting!
And of course if the talks are called off, Trump will have no claim on the Nobel Peace Prize.