In a nutshell...politics.
no, it is far more complicated than that...it would take an hour to explain everything, but in short, a couple of things would be cost and practicality...for one, many of these engines could not be converted to a "clean diesel" design, so you would need to develop and entirely new engine, which would not only be extremely expensive, but also much more complicated...the increased maintenance costs would add to the costs of machines themselves and the running of them...that means, anything that uses these (transport trucks, construction equipment, etc) would cost more, which of course would trickle down to the consumer, so everything would increase in price, everything...secondly, the "practicality" point is two fold...you would be spending huge amounts of money and resources (time and investment) into "improving" on something where the end result will have little impact on the "big picture"...as well, it is quite possible that the "improved engines" don't perform as well, reducing their intended effectiveness to do the job they were originally designed to do.
in short, by forcing those same requirements on them, would be political, rather than practical.
for reference purposes (proving my point about the fact it would make little improvement due to the sheer numbers difference between "passenger vehicles" and heavy equipment), here are the numbers from 2005 for Canada:
Passenger cars and trucks (light and medium): 18,314,103
Heavy Equipment: 294,193
light and medium vehicles account for 98.4% of the vehicles in Canada, meaning it makes no sense to slap silly restrictions on the heavy vehicles.
http://oee.nrcan.gc.ca/Publications/statistics/cvs05/chapter2.cfm?attr=0