Emissions requirements did drive the change. Starting in 2004 the US introduced 10 "Bins" of differing emissions compliance levels. Bin 1 is the cleanest (zero emissions) and Bin 10 is the dirtiest. Over time increasingly cleaner levels are required of light duty passenger cars. Bins 9 and 10 were no longer permitted as of the 2007 model year. Bins 6, 7, and 8 were no longer permitted as of the 2010 model year.

The om648 E320 CDI is a Bin 10 vehicle, hence not legal for sale in the US after 2006. The om642 E320 Bluetec is a Bin 8 vehicle, so OK for sale from 2007 through 2009 model years. The e350 Bluetec (with DEF) introduced in 2010 is a Bin 5 vehicle (which also happens to have been the California standard all along).

For comparison, Bin 10 permits 0.6 grams/mile of NoX, Bin 8 is 0.2. Bin 10 permits 0.8 grams/mile of particulate matter, Bin 8 is 0.2. CO and Formaldehyde standards are the same between the two. So there are significant reductions in NoX and PM emissions going from the 648 to the 642 - approximately 70%.