economies of scale and price of raw materials.
the more batteries made..the cheaper they get.
the cheaper "stuff' they're made of..the cheaper yet.
several new battery technologies are in the Lab as we speak that meet both requirements.
the problem is commercialization
that takes money and time.

I predict solar boats in the next few months
(I'm building one)
and solar other stuff real soon.
the cheaper, lighter, more powerful, (etc) the faster it will spread.
and feed upon it's own success.
think 'moores law'...on steroids.