Any carbon-based fuel ultimately depends on a lot of capture infrastructure to be carbon-neutral. In the short term, it's hard to see a path to viability. Ammonia is easy to synthesize, ingredients are literally water and air, liquefies at achievable pressure, and can be directly consumed in a fuel cell or combustion engine. It's corrosive, but only to certain materials (late transition metals and aluminum), and toxic, but only in high concentrations (the body naturally contains a few ppm of ammonia at all times). It's biodegradable, so you don't worry as much about spills. It's flammable, but not extremely so like hydrogen.

> You still have to deal with a lot of specialized infrastructure and engine design.

Is that really such a downside? Ports are full of specialized infrastructure anyway, and the costs of engine design can be amortized over many engines to be quite minimal overall. Once you've done the design work an ammonia engine isn't more expensive than a hydrogen or diesel engine. And the upsides of ammonia are substantial: fairly easy to handle, quite energy dense and most importantly it is already produced worldwide in huge amounts.

> I wonder why ammonia became the solution that went ahead to prototype and now appears the top candidate for near future low carbon alternative?

You should probably check who would control a larger part of the ammonia supply. I can see few other reasons for putting large quantities of something so unforgiving as ammonia on ships.

> I wonder why ammonia became the solution that went ahead to prototype and now appears the top candidate for near future low carbon alternative?

NH3 has no carbon in it. It has the advantage of not producing CO2. This is a good advantage for public communication.