It's not exactly true, the western front was far from static until the battle of the Aisne and the Race to the sea, and it opened up again in 1918. Meanwhile the eastern front was quite mobile.WW1 was very static war and probably game developers fear from that the WW1 game will be boring after a few runs.
An easy way would be to have a delay between giving an order to an unit and it executing it, which would depend on tech, doctrines and various other factors.In WW1 communication between units was slow and i don't know if it's possible to model that.
At the very least provinces should be much bigger so coordinated movements in great details are not possible.
well. there's a new Beta REDUX version that's currently updating the old WW1 mod. as that mod is more or less dead, due to the developer of that mod decided to flat out begin to work on his very own WW1 game instead.