Residents of Greenland currently enjoy:
Free Health Care
Free Education from primary school up to a Master's Degree
A nationwide minimum wage equivalent to $20 USD
Five Weeks of paid vacation a year
Up to 52 weeks of paid parental leave
Comprehensive social security, unemployment benefits, sick leave, maternity leave, and pensions.
Plus, since Denmark is a member of the EU they can freely travel to and or seek employment in any EU country.
Why would any rational citizen of Greenland think joining the United States is a better option than what they have now?