It's hard to talk about combat in WW1 without talking about trench combat as it played such a large part in the war. In WW2 on the other hand trench warfare seems to have almost died off. So I have to ask what advancements/ changes in the years between world wars led this to be so, or is it simply a misconception?
Motorized armor.
In 1918, at the Battle of Amiens, the allies rolled out hundreds of clunky but heavily armored tanks. Artillery has a harder time aiming at moving vehicles, and machine gun/rifle bullets obviously weren't stopping them. They rolled over the German lines, gunning down anything that moved, and once punched through, would allow the infantry to follow. They rolled through 17 miles and into open countryside for the first time in nearly 4 years. The Germans recognized this day as the day that broke the German army. The armistice was signed a few months later.
So, advance the technology in motors, armor, guns, and tanks overall, from 1918 to 1939. Put big turret guns and other small arms into the tanks themselves, and in peacetime, practice driving with them and firing with them. Pretty soon you have a new cavalry corps that represents an army where offensive capabilities now outstrip defensive trench warfare designed to stop mass infantry assaults.
The French in 1940 were expecting a repeat of trench warfare. What they got was a bad experience in motorized Blitzkrieg, as the Germans cut the defense forced in northern France in half, surrounding the northern salient around Dunkirk and pressing south against the remnants of the broken French army. Add in airborne dive bombers, and you have an entirely new way to wage warfare where defensive, static trenches offer you little advantage anymore. Times had changed.
Are you asking for WWI to WWII specifically? Because trench warfare in general hasn't necessarily died out entirely. An example would be the Iran-Iraq war and the currently ongoing Donbass conflict, for post 1918 trench warfare.