Passenger cars are "supposed" to use lubricants that the OEMs recommend. Part of that recommendation is fuel economy, which then dictates the viscosity grade. To keep a consistent message to consumers, 10W30 is the highest viscosity grade recommended (5W30, 5W20 are the others)because it still fits the ILSAC GF-3 requirements and allows the "starburst" symbol on the front of the bottle. By definition, any oil that is a xxW40 is not even eligible for the starburst symbol. OEMs want this system because they can then recommend the use of any starburst oil (and not have to explain viscosity or API performance levels) and meet their obligation to the EPA to recommend fuel efficient oils.
Part two of the starburst is emissions system protection. Current oils have a limit of 0.1% phosphorus, even though current levels of phos has not been linked to catalyst degradation. The current lubricant limits are "formulation" limits (i.e. 0.1% max) because the technical groups that develop lubricant performance requirements cannot come up with an engine test that would determine the appropriate level of phos. Oh well, the OEMs got their way.
Along come diesel oils. While a 15W40 might cause a little cold starting issue sometimes, a 5W40 would not. It will perform the same as a 5W30 or 5W20 when it's cold. The -40 part will provide some added bearing protection, at the slight cost of fuel economy (maybe 1-2%?)
Diesel oils are not bound by the phos limits, so when the next category after API SL/ILSAC GF-3 comes out, they will remain at the 0.1% phos level, even as the passenger car limit will decrease to 0.08%. To me, it's a little more serious that taking the lead out of gasoline; older engines designed for older oils that had 0.12% phos will now be exposed to oils with a lot less wear protection. There has been a lot of debate between the OEMs and the oil industry about whether the 0.08% level is backwards compatible or not, and again, the OEMs got their way.
Will the current level of phos harm a catalyst? No, because even your current catalyst was designed for the current level of phos in oils. What the OEMs are getting is an oil they hope will allow them to use less precious metal loading on future catalysts (or the argument that now the catalyst will last longer.)
Diesel oils are formulated to tolerate upwards of 5% soot in the oil and still prevent wear, sludge, thickening. They are formulated to prevent varnish and ringland deposits under highly loaded, high pressure turbocharged conditions. The additive package is typically nearly twice that of a passenger car oil.
Summary: Delvac 1, Rotella T synthetic, Amsoil, or any other diesel 5W40 has as good cold starting, better bearing protection, worse fuel economy, maintains anti-wear levels, equivalent emission system effects, and has better dispersancy and detergency than a passenger car formulation.
And finally the now-obvious answer to your first question: People don't use diesel oils because the OEMs tell them not to.
