ICE Engines are designed to burn the fuel and air mixtures at high pressures to increase the combustion efficiency. But hydrogen combustion efficiency is already high at 1 atm and at constant Volume combustion, peak pressure may reach around 8 atm. So why compressing the hydrogen and air mixture in Hydrogen ICE engines? If we could avoid compression of fuel and air, we can design much simpler and cheaper Hydrogen engines.
SolarMike's comment basically answers the question, but let me flesh it out a little.
Let's take a typical engine, say it has a compression ratio of 12:1, and weighs 500 pounds, and has a displacement of 4 liters. Now let's reduce the compression ratio to only 2:1. To get the same amount of power output, the displacement now how to be 24 liters. (can you see why? burning one-sixth as many fuel molecules per liter, so need six times more liters). So now the cylinders and pistons have to be about six times bigger as well, so now the car/truck has to be a lot bigger overall to fit this giant engine in there. Also those bigger cylinders are going to weigh a lot more, potentially as much as the entire car/truck, which hurts the performance of the vehicle. Overall it just doesn't make any sense to use this kind of engine. The power density is too low.