Possibly because we're developing to a society where the only people who can afford to have multiple children are either the wealthy and those so poor that they know they're kids can get welfare (or where "fathers" recklessly sire a bizillion children by a bizillion different women). The best the middle class can seem to hope for right now is to have two or so children and try to do the best they can for them. I am an only child myself.
The other option for the middle class, of course, is to buy smaller houses, not insist on living in a trendy suburb, not buying all designer clothing, not buying a brand new SUV, and maybe (God forbid) going without cable television, so as to be able to put more money into one's family.
Of course, Pat, and whoever else has said it, is right about the demise of faith causing a demise in population. Secularization and the marketing/mass media culture have knocked our priorities way out of whack. We are only still ahead of the Europeans because we have a greater vestage of our Christian heritage left. If that ever goes away, we'll go down the tubes twice as fast due to our rampant materialism.
"A comparison between Teddy Roosevelt's America when the WASP ethos was in flower with George Bush's relativistic, "One Nation under Therapy" America is striking."
So true. Even in the Fifties and Sixties that ethos at its best was still evident. In our small town when I was growing up there was an elderly Protestant man, a retired lawyer, who used to shovel the sidewalk in front of his house after every snowfall. Then without fail he would shovel the sidewalk of the two spinsters across the street. He was a good neighbor, who embodied the WASP ethos in the most quiet, unassuming way, a real gentleman.