I think the bigger question is why the US hasn't just adopted all of the 30+ years of testing data that the Swedes have on car seat safety, and apply that to our own car seat and testing standards. The answer to that sadly is a combination of money and turf. Yes, changing the test sled designs is expensive, but it's a one time expense and should be rather quickly absorbed. The huge expense will be the one borne by the car seat manufacturers, and I guarantee that they are spending GOBS of money lobbying to keep those standards and testing methodologies out of this country. Why? Because quite frankly I'll bet you dollars to donuts that a good number of seats with huge sales numbers will fail those tests. THAT will cost the seat manufacturers a fortune, and THAT is why they spend so much money on lobbyists to keep Swedish, and even Canadian standards out of the USA. You can give me lip service about how it will make car seats too expensive for the poor here in America, and I'll just point you to affordable seats sold in countries that do more stringent testing with more realistic test beds and more realistic crash scenarios. If need be, then we just give vouchers for all to partial payment of seats via sCHIP, WIC, or Medicare/Medicaid.
Remember, these car seat companies are not non-profit organizations, and they already produce seats for Canadian, European, and Swedish standards. They already produce affordable seats that adhere to those standards as well. They just don't make near as much profit as they do making seats for countries with more lax standards. Their testing, design, and material costs are lower here, so they make more money on a seat that sells for roughly (considering exchange rates) what a comparable seat sells for in countries with more realistic testing procedures. It's that simple.
As for turf, it's pretty obvious that our government has always been reluctant to accept scientific safety data from other countries because we didn't think of it first. Well, that and lawmakers risking contribution dollars and losing elections because they ticked off some part of corporate America by adopting something that might be financially onerous to them. Regardless of the fact that said scientific safety data would save lives or the environment. Quite frankly, we American's created the bed we are lying in right now, and until we the voters finally get tired of getting screwed by both political parties and band together to make sweeping changes, it will never change significantly.