The problem with WW1 games, especially ones in the trenches, is that it's very grim and depressing and doesn't make for a very entertaining experience.
Toy Soldiers is an excellent RTS/Tower Defence game set in WW1, but despite all of the pieces being toys and exploding into little plastic bit and cogs when destroyed, it's still terrible depressing and grim and is a stark reminder of the senseless waste of life that was the trench warfare and idiotic infantry tactics of WW1.
Perhaps that makes it a more powerful game, because where in normal Tower Defence games, channelling the enemy into a a narrow field of fire provides a sense of jubilation and victory, doing the same thing in Toy Soldiers, using barbed wire to channel the German infantry and cavalry into the overlapping fields of machine gun fire whilst gas and flame weapons mop up any survivors, the same type of decisive victory wasn't really entertaining at all.
Another old WW1 game I starkly remember was Wings by Cinemaware. At the heart of it, it was a dogfighting and aerial combat/shoot-em-up game, but a combination of the setting, music and detailed diary entries compiled by your character(s) meant that the horror of the situation and the horrible conditions on the front line were inescapable.
It's a difficult subject and setting for a piece of interactive entertainment to tackle. If you make your game realistic and treat it with the respect and reverence it deserves, then you run the risk of your game being too depressing and not entertaining.
On the other-hand, if you make your game too entertaining, or remove it from the setting enough to lose the emotional impact (either through narrative ignorance of the subject matter, or a fantastical alternate history), then you run the risk of making light of such a horrible period of history and also raises the question, why bother to set your game in WW1 if you're going to ignore everything that made it WW1?
It is entirely possible to make good games set during WW1 but not on the Western Front, perhaps from the perspective of a different branch of the armed forces or somewhere else in the world that was at war, and it is also possible to have a entertaining game that features trench warfare like WW1, but isn't (e.g. the opening trench levels of WH40K: Fire Warrior), but a game, especially a shooter, that is set in the trenches of WW1 and features the level of visceral realism and historical accuracy that we've come to expect from such a shooter, well that isn't really good setting for an entertaining game.