Europe's diesel emission standards have been a bit less stringent than EPA's and much less than California's, until very recently. Current standards are roughly equivalent to Tier 3 1/2 for on-road stuff. It can be done without SCR & fluid, and with relatively minor tweaks even VW can probably fix their small engines to do that, but it's easier to meet and requires fewer performance compromises if done with the catalyst. EPA/California Tier 4, for practical purposes, is a catalyst game because it is the only way most (even VW, for the larger engines, and as we're now seeing maybe also the smaller ones in Real Life) can do it.

Larger diesels (especially large stationary and locomotive/marine) have, in general, had less stringent emission standards that on-road; there are, of course, many fewer of them than on-road, and they last a long time. But Tier 4 has been around for years for stationary engines, eventually arrived for locomotives, and California has a program to push cleaner engines into watercraft based in the state too. Even a Tier-3 645 or 710 is far cleaner than many existing engines in the marine sector.

When you're pushing the limit on relatively inexpensive emission reductions from on-road sources and industry, and still have significant violations of health-based air quality standards (many places in CA, but especially San Joaquin Valley and South Coast), you have to start looking for anything you've missed ... and the reductions start getting really expensive. Remember that "health-based" bit - non-compliant VWs, and coal rollers, *do* kill people due to their excessive emissions, just not (usually) right away.