What was the reasoning behind deploying this as main tactic? Was the tech available not able to support an offensive military doctrine?
WW1 started out as a highly fluid and mobile offensive in france. The germans and the allies were fighting a war of maneuver. After the germans were halted at the Marne river both sides were pretty much out of steam so naturally fell back into a defensive position after securing their flanks, all the way to the English Channel. Trench systems have been used since cannons were invented but WW1 saw the widespread use of machine guns which changed the nature of defensive positional warfare. Now a couple well sited and dug in machine guns covering each others arcs could stop an entire division in its tracks.