Energy density is a big issue. Bunker oil is nasty stuff, but weight/volume/energy is very hard to beat. And it's redicoulsly cheap, thanks externalities of local and global environmental impact/risk not being properly priced in. But ammonia-hydrogen at least approaches the ballpark of diesel fuel iirc (also much less concentrated energy - but a more "natural" replacement for toxic bunker oil, which is finally being banned due to the pollution from regular use and spills).
Some numbers here (Esp concerning energy density etc):
