A guy named Carl T. Bogus (he’s for real) speaks:
One striking difference is that the iconic conservative works are about ideology. By contrast, the most influential liberal books of the era are about policy issues. Those works are Silent Spring by Rachel Carson (1962), The Other America by Michael Harrington (1962), The Feminine Mystique by Betty Friedan (1963), and Unsafe at Any Speed by Ralph Nader (1965), which helped launch the environmental, anti-poverty, feminist, and consumer movements, respectively. Some prominent liberal books of the time were about ideology — such as The Vital Center by Arthur M. Schlesinger Jr. (1949) and The Affluent Society by John Kenneth Galbraith (1958) — but these are exceptions to the rule.
…Conservatives have big appetites for ideology; liberals don’t. There are, of course, taxonomies of conservative schools of thought. People on the right classify themselves as libertarians, neoconservatives, social conservatives, traditional conservatives, and the like, and spill oceans of ink defining, debating, and further subdividing these schools of thought. There is no parallel taxonomy on the left.
Addendum: Arnold Kling comments.