Speelights - why not L ion?

Suntan wrote:

Standard AA sized cells are 1.5V nominally (rechargable NiMH are about 1.2V) but one cell of a LiIon battery has a voltage much higher than this.

As such, it would be very difficult to create a standardized LiIon batt. That isn't to say that a company couldn't make a speedlight using LiIon, but most companies choose to make their speedlights compatible with the ubiquitous AA standard.