You need to show a long term relationship and commitment to make a lasting marriage.
Again, the concern is she will jump ship and disappear or be abandoned and unable to support herself.
"A K-1 visa is a visa issued to the fiancé or fiancée of a United States citizen to enter the United States. A K-1 visa requires a foreigner to marry his or her U.S. citizen petitioner within 90 days of entry, or depart the United States….Wikipedia ".