Sorry to hear about this – you should have received an email and a message with more info in Creator Studio as to why you were demonetized. Take a look at the detailed resources and note that you can always reapply for the YouTube Partner Program again in 30 days.
We've received an update and can confirm that the monetization was not approved because of less public watch hours than required–Creator Studio shows public & private combined, but you need 4k public hours alone. More here: https://yt.be/help/YPP-review