Millions of people have wanted to come here, but not that many employers were hiring foreigners until around 1998 and the dot-com boom. So family-based categories have been seriously retrogressed for a long time, but EB categories have mostly remained current or had nothing more than slight retrogression until recent years.
In 1998 and beyond, the employers increased their hiring of skilled foreigners and the US government more than doubled the H1B quota for a few years ... so the natural consequence of that was an eventual increase in employment-based green card applications. That was offset somewhat for a few years (I think 2001-2004) by recapturing unused green cards from prior years. Once the recaptured green card numbers ran out, serious retrogression kicked in. But they went overboard with it, leaving some 11,000 green cards unused for FY 2006, and then having to do a big jump-forward of the dates in June 2007 followed by the "all current" fiasco in summer 2007 to use up the remainder of the quota after having approved very few up until that point of the year.