The answer is simple... The system is not set up to encourage change in the maritime shipping sector.  The technologies exist, but It seems that one of the biggest elements holding us back is the lack of financial incentives for ship owners to convert to green energy... This article explains the situation well: 
.www.climatechangenews.com/2016/08/30/why-ship-owners-see-little-point-in-greener-vessels/