Unfortunately, they were limited by the technology of the day - not the lack of mechanized forces, but the lack of portable radios. Charging across the no-man's land, while high in casualties, usually worked for getting an initial penetration of the other side's trench system, but without radio to coordinate reinforcements, artillery support, and a further exploitation of the breakthrough they would quickly fall to counterattacks. And then the bloodiest bit of the offensive, the initial breakthrough, would have to be repeated again.
Yeah i recall reading about how they would attempt to roll telegraph wire out as they advanced, only to have it snag on something or get cut by artillery blasts.
I think it did relate to mobility and supply chains as well. But, yes, the bottom line is that costly long-planned breakthroughs invariably ended up getting bogged down.
