There are cars with petrol engines having 1,0 l or even less engine displacement but I have never seen a car with diesel engine having less than 1,5 l. Are there technical reasons that diesel engine cannot have less than about 1,5 l?
There are model diesel engines, so technically it is possible. My guess in the case of car engines would be that it is about economics. Diesel engines are more expensive to manufacture (because they need to be stronger to withstand higher compression), but you earn that back in their better fuel efficiency and lower fuel taxes if you make enough kilometers. People who buy small cars are more likely to not make a lot of kilometers per year, and more likely to care more about the car's initial sale price compared to fuel costs.
Diesel engines use compression to ignite fuel. Compression ignition requires higher compression ratios than Otto cycle engines. This results in parts that have to tolerate higher stresses, which means heavier parts.
If you try to shrink a diesel engine down, you'll have small cylinders and everything else will be heavy (even the engine block). It's just not an efficient setup—lots of work goes into moving those heavy parts!
Also, the main benefit of diesel is higher torque at low RPMs. A small engine is probably in a small vehicle. You don't need lots of torque to get a small vehicle going.
Ignition lag in diesels makes it difficult to run them over a range of rpms that peak out above about 4500 rpm. You can tinker with them to run just at higher rpms, but not to run over the wide range of rpms required of a vehicle diesel. The little 1 liter gas jobs can be perfectly driveable at 7000 rpms or more. Hence they don't need to have as large a displacement.
Honda's new little aluminum diesel apparently tops out at 4000 rpm.