Why are more powerful tools using higher voltage?
Matt already explained that using a higher voltage you'll have a lower current for the same power rating. This means thinner and less heavy wires, which means savings (copper is expensive). You may have to pay attention to better insulation, but that doesn't outweigh the advantage mentioned.  

It's also much easier to place cells in series than parallel. When placed in parallel the voltages have to be exactly equal, otherwise you'll have high currents running from one cell to the other, causing big power losses and reducing the cell's life.