The reason is humanity is simply at the limits of understanding in battery science.
Lithium-ion is very promising, but at this point we are making hundreds of small tweaks, over and over which steadily improve performance. Drive is for higher capacity rather than higher amp drain as electric cars need a certain range to gain mass market appeal - and they use more batteries than any other devices.
All the research is on the nano-meter scale. Playing with the surface structure to enable a more efficient flow of lithium ions that is.
It will most likely continue to be improved at a slow and steady rate until a major break-through is found in lithium-ion, or in another battery chemistry altogether.