The why comes down to inertia both in the flywheel and corporate thinking.

I can show you all sorts of cost benefit analysis done in industry and if you get the tech right you'll come out ahead. But no one wants to guess wrong and be left holding the HD-DVD bag. While high pressure storage and sourcing hydrogen in bulk can be an issue. The simple changes to run a diesel in hydrogen allows a safety blanket for the MBA types making the call. Since they can be reversed easily.

Also the amount of rotational momentum in a ICE is quite useful in starting large hydraulic pumps etc. Totally can be overcome as well but just a point.