First, I think your understanding of prior conflicts is incomplete. Trench warfare had been a definite factor in many prior conflicts. There was trench warfare in the Crimean War. There was extensive trench warfare at the end of the American Civil War, where European observers could plainly see the effectiveness of the tactic (if they did not already understand). Closer to 1914, the 1905 Russo-Japanese War was one giant trench warfare conflict. Both Balkan Wars had loads of trench warfare, and even the Italian invasion of Libya had trench warfare elements even though the defenders were largely lightly armed irregulars and militia. The Boer Wars as well had both sides using elements of trench warfare. Indeed, I believe the examples where trench warfare is NOT present are the exception to the rule.

The reasons that that style of conflict faded away chiefly boil down to improvements in transportation technology and communications technology. As communication improved (portable radios, in particular), the ability for an attacker to coordinate adjustments on the fly during an assault resolved many of the existing disadvantages attackers had. Suddenly, instead of reach a preset goal and waiting for new instructions for crucial hours a well positioned unit could be used to its advantage within minutes. Conversely, if a sector of an attack stalls and is not reaching goals in a way that would expose other elements of an attack to critical danger, the risk of disaster can be averted much easier.

Taking it even further, portable radio equipped units trained to direct and correct artillery fire could now call down a battery onto targets that either had not been identified for bombardment before the attack or perhaps had survived the previous shelling. The same units could then provide real time feedback on shot falls for the gun crews to correct the aim. This makes hard static defense quite difficult to maintain for long periods of time. Well directed batteries can just blow up your stuff faster than you can build it.

The defender already had most of the advantages with buried telephone lines. They could call for critical reinforcements or artillery support (which would be to pre-targets spots) and give targeting corrections as well, all on regular telephone wires.

On transportation, trench tactics worked partially because logistics were limited. You needed railroads to move, feed, and supply a force of any significant size. Then factor in other basic geography (rivers, lakes, swamps, forest, mountains, etc) and this meant assault points were pretty predictable just by looking at a map. An attacking force needed nearby railroads and geography that didn't prohibitively favor the defense, and then the attacker generally needed a long buildup of supplies and men that was obvious to the opponent. Then even if you have all that, your attack can only move as fast as livestock can pull a howitzer and/or a laden supply wagon. Sure, at first your attack will move as fast as soldiers can march, but that will end in a few days with the infantry exhausted, starving, low on ammo, standing in the open, and with no artillery support. Beyond all that, given even WW1 logistics an attack could only go as far as wagon drawn supply could go before the livestock ate all the food they were carrying, which was about 100 miles from the railroad. Shot short term attack moves at speed of man, medium term moves at speed of burdened livestock, long term moves at speed of building new railroad.

With the coming of reliable (or at least "reliable") combustion engine on a mass production scale, this picture changes completely. You motorized supply greatly extends the speed an attack can move at every time scale (at some point you do still need railroad support, or a port). Motorized logistics generally don't eat the supply they are bringing to the front, unless they are supplying other motors. They are also faster than burdened livestock and more space efficient on a given area of road, being able to carry more. You can also use them to shuttle manpower around independent of the rail lines, making your deployment of force less predictable. With less predictable force deployment for an offensive a defender has a harder time realizing the economies of force that were available in WW1. The first War's trench systems worked only because the defender was able to lightly hold those areas they "knew" were unlikely to be attacked (due to the above) while they strongly held those areas that were the obvious targets. They could not strongly hold everything, which is what would be required if the attacker has the freedom to choose.

The response to a system where the attacker is motorized and free to choose the point of conflict is defending in depth, where the front line is mostly a screen to sound the alarm and delay the attack and the real defense is deployed in depth. The depth can also be motorized and move quickly to the battle, using wireless communication to coordinate with command HQ...

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but then you just end up with a much more fluid form of combat that looked a lot more like mid to late WW2 and most conflicts since.