Since most WWI battles weren't fought in parking lots, this tank was only slightly more ineffectual than the tank it replaced, thanks largely in part to that massive appendage on the front, which had a tendency to dig straight into the ground and get the whole tank stuck.
Whenever the tanks surged across the battlefield, they would quickly and comically stumble over craters, mounds or just about any slight change in elevation, and since the gun couldn't rotate or elevate at all, the only thing it could do was shoot at the ground. So while it may have been useless against other vehicles and humans, the tank was very dangerous against enemies like dirt and mud.
"Shit. It's a hill -- get out the white flags."
If they ever did make it all the way across, they got stuck when their noses dug into German trenches, because they had slightly widened them specifically to fuck with these tanks. Yeah, that's it. No super gun, no giant antitank flamethrower, no giant-ass mines, just a little extra digging and this massive weapon became obsolete.