Some may argue that Nella Larson's Quicksand fails to adhere to the Modernist genre. This said, two distinct things about Larson's novel make it Modernist.
First, the time it was written in identifies it as Modernism. It was published in 1928. The Modernist period spanned 1860 through 1940 (though some critics dispute these years--some push it as far forward as 1960).
Second, Modernism acts as a renouncement against Realism. Realism showed life as it really was. Modernism, on the other hand, depicted life through the experiences of mankind. It depended upon the insight and interpretations of the individual (regarding his or her world). Therefore, given that the novel illustrates Helga Crane's interpretation of the world around her, the novel upholds its Modernist association.