Why was the bayonet ineffective during WW1?

One reason why the bayonet was ineffective in World War I was because of the rapid growth of technology in the field of weapons.

The bayonet was a remnant of a previous time period in warfighting.  Its purpose was to get close to a soldier and stab him as he approached.  For 19th Century tactical approaches such as grid- like soldier formations, the bayonet was useful to stab and advance.  However, technological advances in weaponry emerging in World War I had outstripped the bayonet's usefulness. For example, soldiers began to use the machine gun.  This allowed soldiers to fire multiple rounds of live ammunition at oncoming soldiers.  A soldier approaching with a bayonet would be neutralized from a distance through a machine gun which could be hundreds of yards away. The use of grenades was a similar means of neutralizing the bayonet.  In both instances, the threat of a solider with a bayonet could be addressed from a distance.  The bayonet's need for close proximity had become a liability.

Another technological advance that limited the bayonet's effectiveness was the use of armored tanks.  The bayonet was not very effective against these "landships."  The use of airplanes in World War I was another advance that made the bayonet ineffective.  The soldier wielding a bayonet was no match against an aerial attack.  Finally, the bayonet proved no match to chemical warfare.  The use of chemical weapons such as mustard gas made the bayonet worthless.  By the time soldiers approached with their bayonet, the gas would have already scorched their lungs or caused blindness. Technological advances made the bayonet largely ineffective in fighting World War I.