There's no question that you can generally get more "miles per gallon" (or km/l) with diesel than with gasoline. The "energy content" that can be extracted form a gallon of diesel fuel is nearly twice the corresponding amount in a gallon of gasoline.

In addtion, most diesel engines rely on "compression ignition" and require compression ratios of at least 14:1, with 16:1 or a little higher being fairly common. Higher compression before ignition allows getting more energy out of each piston stroke more efficiently. It also, unfortunately, results in higher combustion temperatures which produce lots of NOx polutants which require very expensive exhaust cleanup. And most diesel fuel contains higher sulphur than gasolines, so SOx pollutants have also been a much more serious problem for diesels than for gasoline engines.

The same high compression used in diesels can't be used with gasoline because the gasoline "explodes" instead of burning (i.e. "knocks"), and destroys the engine. In the past "additives" could be used to increase the octane number for gasoline, but the ones previously used have been found to produce very high levels of exhaust pollutants, and most are now prohibited, while the few new additives found are very expensive.

Not too long ago in the US, diesel fuel was considered almost of "byproduct" of gasoline production, and usually sold for about 80% or less of the price for the same volume (gallon) of fuel. If the fuel costs half as much and takes you twice as far, that's a 4:1 "cost advantage." Due to increased use of diesel and declining consumption of gasoline (at least as claimed by the producers) diesel in the US currently costs almost twice as much per gallon as gasoline in some places. While the mpg (km/l) difference still exists, doubling the relative price of the diesel vs gasoline just about wipes out any economic advantage. (Maybe that's the real objective of the fuel suppliers' "pricing strategy"?)

It's hard to say whether it's "market driven" or actual cost driven, but in general it's still pretty much true that a diesel engine that replaces a gasoline engine in a given vehicle costs about double the price (of the engine) at the point of purchase, and in most "personal vehicle" applications where the swap makes sense the diesel also requires a different transmission that's about 50% more expensive. Initial purchase price probably has quite a bit of effect on the relative popularity of gas vs diesel in US markets, although I can't say whether that's as true everywhere.

Q - Cummins Diesel signs are still up on the maintenance shops in my town, and so far as I know both Ford and Dodge light trucks still use them. GM has generally made their own, although recent options may allow other "brands." Cummins is still a favorite in heavy trucks, although many of the vehicle builders claim to build their own engines.

Onan, another name, was absorbed by Cummins some time ago. Perkins used to be well known in Europe and I don't know whether it still exists. It was never common for vehicles in the US, other than for a few farm tractors prior to about 1945(?) but was used quite a lot for "stationary engines," and was pretty common on oil well pumps here some time ago. Several "European manufacturers" have published brags about "their new engines" but it's uncertain whether they actually build them themselves or just "brand stamp" what they buy from someone else who builds to their spec.

John