When I was younger, it seemed the U.S. Men's volleyball teams (and the women's teams) were usually at or near the top of world competitions. Nowadays, it seems that U.S. Men's volleyball has slipped a bit on the world stage. One reason I suspect for this is that many colleges and universities - including Division 1 schools that are competitive in many other sports - don't have men's volleyball teams, even though they have women's teams. Without many opportunities to play and compete at a high level in national competition, the U.S. Men aren't prepared for the top teams in world competition. I've also noticed that many high schools (both public and private) don't have boys' volleyball, but do have teams for girls. Why is this?
This is a result of Title IX, part of the Educational Amendments Act of 1972, the US federal legislation which outlawed sex discrimination at federally funded institutions. One of the tests for Title IX compliance in sports is that the amount spent on athletic scholarships available is proportional to the number of students of each sex; as almost all institutions have a close to 50-50 sex ratio, this therefore means that each sex must receive an equal amount of athletic scholarship money.
The other factor here is (American) football. As effectively every institution runs a men's football team but none run a women's football team, this means that there must be more money allocated to women's scholarships outside football than is for men in order to maintain the required balance. Volleyball is one of the primary sports which gain these extra women's scholarships, although other men's sports have come under pressure in recent years.