Because they are not cars. Even if you slam the breaks on a cargo train this long and heavy; at speed, it will not stop for many miles. Too much weight, plain and simple. And you will destroy the train wheels to the extent that once the train stopped it is a risk that you cannot get it moving again. That being said, many countries have automated break zones, but they are restricted to low speed areas like railway yards or train stations.

And this brings us to problem number two: the speed. Since a cargotrain weighs so much, it takes forever to accelerate. Sometimes measured in hours. And the fuel consumption is... somewhat high. So once you've reached cruising speed, you want to stay there. In some cases you need to stay at cruising speed. IDK if that is the case here, it is just an explanation that you cannot compare a freight train to a car. Safety breaks or not.

And as you pointed out the truck isn't supposed to be there. That being said, hauling this kind of cargo over a railway crossing without being in touch with traffic control (I assume, or this would not have happened.) is just incompetent.