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Diesels like cold air.
Lets do a thought experiment. Imagine you have two identical cylinders. One cylinder with 100 grams of air and a cylinder with 110 grams of air. For now lets assume the starting air temperature is the same. Now imagine we add the same amount of heat energy to each cylinder. Some of the energy is going to be transferred through the cylinder wall to the coolant and the rest is going to be transferred to the air. It stands to reason that the air temperature of the cylinder with 110 grams of air will not get as hot because the heat energy is heating more mass. Since the cylinder with 110 grams of air will not get as hot less heat energy will be transferred through the cylinder wall to the coolant and we can extract more of the remaining energy from the air.
Advantage cylinder with more mass in it
Now imagine you have two identical cylinders with 100 grams of air in them. Lets assume that the air temperature in one is 20 degrees C less than the other. Now imagine like before we add the same amount of heat energy to each cylinder. Just like before some of the energy is going to be transferred through the cylinder wall to the coolant and the rest is going to be transferred to the air. It stands to reason that the air temperature of the cylinder that was 20 degrees C lower than the other one won't get as hot because we are heating the same amount of air with the same amount of energy. Since the cylinder that was 20 degrees C lower than the other one won't get as hot less heat energy will be transferred through the cylinder wall to the coolant and we can extract more of the remaining energy from the air.
Advantage cylinder with colder air in it
Now with a cold air intake you get more air in the cylinder and its colder, thus its a win win with a diesel.
This holds true until you cool the air down enough that fuel won't ignite due to temperatures being to low a top dead center. People may argue that since diesels have trouble starting in the winter engines are close to that already but this is not the case. The main reason diesel engines have trouble starting in the winter is the fuel is cold and doesn't atomize well because its more viscous. That is why they thin diesel fuel out in the winter.
There are other reasons why a cold air intake is better as well but these are the two main reasons that are easiest to grasp
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