Environment plays a huge role in the development and maintenance of IQ. And socioeconomic status (SES) can greatly affect the conduciveness of a child's environment to fostering IQ development. Accordingly, since more and more children are being born into low-SES households, it isn't surprising to see average IQ scores in decline.
Consider the following tidbit from the most recent national census: 50% of all babies born in the country right now are born into low-SES households (as defined as households dependent on governmental assistance, like food stamps). Forty-one percent of all babies born in the country are born to unmarried mothers (single-mother households being highly correlated with low SES).
The conclusion isn't that our generations are getting dumber, per se; it's that our generations are being born into poorer and poorer households. And the deck is pretty significantly stacked against someone born into a low-SES household.
Now, there's debate about causality in all of this, i.e., whether the existing population is actually getting poorer, or whether the poor are simply outbreeding the well-off. Based on census data, it seems to be a little of column A, and perhaps a bit more of column B. Either way, the younger generations are starting off on worse footing. And that leads to lower IQ scores and decreased future prospects.