I would imagine Forcesabers differ fundementally from lightsabers in that the weilder seems to be powering the actual blade WITH dark side use of the Force, not energy from a power cell like a lightsaber. It's not just Force activated like the later lightsaber option, but actually "Dark Side" powered, with the ebon crystals focusing the dark power into a blade. This would make it alien enough to not be a standard lightsaber, while still resembling and influencing the later development of lightsabers. And obviously finding a power source to replace the furious energy of the "Dark Side" would be difficult for the much-less advanced society of the Tython system or ancient Republic, hence the generator-based siege weapons, then later belt powerpack lightsabers. EDIT: It also potentially opens up the Sith having Forcesabers prior to the arrival of the Exiles, since they stole so much Rakata tech after forcing them off Korriban. Which then allows the Exiles to work on cordless lightsabers separately from the Jedi, which could explain why they seem to have cordless lightsabers sooner than the Jedi in the more recent EU. Huh.