It is very different to be able to demonstrate a principle in physics (such as flight, or accelerating a piece of metal with electricity) and building a practical application of that principle. In many ways the time it takes between those two points is likely a good indicator of just how difficult it is to reduce the principle to practice.
You yourself mention super capacitors, another comment mentions the rails, then there are the switching devices themselves, the wiring harnesses, the control system. That is why I'm pretty awestruck at the fact that they were able to make it work and get this close to being something they can field test.
[1] "About 100 years ago, an Englishman, Sir George Cayley[2], carried the science of flight to a point which it had never reached before and which it scarcely reached again during the last century." -— Wilbur Wright, 1909 --- http://www.ctie.monash.edu.au/hargrave/cayley.html
On ships and airplanes, you can replace internal weapons bays which have substantial storage capacity. And the batteries (this is particularly an issue for airplanes) aren't terribly mass-dense either. And you can use the vessel's turbine-powered electric generators to recharge the batteries from fuel (this reduces the range, but hey, tankers).
And in the fog of war... Who's to say that a DD won't be able to close to railgun range of an enemy warship? Life's a lot easier when you can stay below the horizon, after all!
[1] https://www.zerohedge.com/news/2018-12-25/china-selling-hype...