They are run by people who are more interested in publishing important work than in making a buck. In fact, some are not for profit enterprises. Some survive by the good graces of a university, by donations, or by grants. Some are entirely labors of love.
Small presses are our best example of the freedom of the press in America.
Often, a small press will speak for an underrepresented group. For instance, Alyson Publications focuses on books for gays, lesbians, and their children. Red Crane Books explores the cultures of Native Americans and Hispanic Americans. Copper Canyon Press has been publishing poetry from new voices exclusively for 25 years, and started as a hand letterpress operation in Sam Hamill's kitchen. Calyx Books publishes works by women from all sectors of society.