Is the grand compaign run only playable with Sparta ?

There is no rule on it, so probably it's either coincidence or preference of players that both campaigns on the board are Sparta.

Hope it helps

It all depends on your desire. In fact, a section was made for each faction here. Personally, I set the record on Sparta only because it was also set on Sparta before me and as a result there was some competitive interest

you can complete any campaign for any faction, and that mileage will be added to the appropriate category