Someone will be along in a moment to tell me all about embrittlement and blue hydrogen
Why ask the question if you already know the answer?
The reason it hasn't taken off is because it's a fundamentally very difficult technology to safely build. Embrittlement is a fact of physics, and it's extremely difficult to design around, especially at scale.
And the fact that there is almost zero global capacity to manufacture green hydrogen means that there is little point in subsidising it from an environmentalist point of view.
Hydrogen will have its uses, maybe in niches like aviation fuel where requirements are very specific and it's possible to exercise much tighter control of the infrastructure chain. But it's just not a competitive technology for replacing petrol and diesel in general purpose road vehicles.