Why wasn't there widespread and long-term trench warfare during World War Two like there was in World War One? Was it a result of technological advancements, military strategies changing, an aversion to trench warfare by military commanders during WW2, or something else entirely?
Hey, I answered this precise question two days ago. You can see the answer here.
The TL;DR is that what we know of as 'trench warfare' only existed because both sides, more or less, wanted it to exist. The Germans wanted to deny the French and British decisive action and wanted to hold them off while they dealt with the East and the British and French realized that they had the Germans totally encircled while also outnumbering them and with significantly more material resources so they felt attrition was a viable option.
The French actually went into WW2 hoping for the same; they realized they and the British combined outnumbered the Germans and they could win through attrition. The hope was to set up trench networks in Belgium, wait until about 1942, and then perform a massive offensive. However the Germans were having none of it; they wanted decisive and quick victory and it was possible for a slew of reasons that I go over in the above post. An extreme TL;DR of these things would be; in my opinion in order of importance:
Increased proliferation of firepower; rather than Battalions having at most 2 machine guns they had hundreds allowing every Platoon of 60 to have that kind of firepower. Add into that Platoons having quite literally dozens if not, at times, hundreds of grenades and infantry became an extremely potent force. This is my #1 because even before everything else after this this caused the defensive 'stalemate' of 1915 trench warfare to break down.
Radio allowed operational tempo to stay high. Quite literally overnight the world went from using carrier pigeons and permanently laid telegraph wire to universal wireless radio. It was the difference between the message delivery time being averaged in days to the message delivery time averaging in seconds. A Division General could get a Platoon to halt their advance in under a minute or tell them to pick it up again.
Increased sophistication in AFV's (armored fighting vehicles), APC's, trucks/jeeps, and aircraft allowed for a more mechanized style of war. Even though the German army in Barbarossa was still using horses for over half of its logistical needs (I've seen numbers as high as 70% purported on /r/AskHistorians!) it still created a system which made entrenchment a less viable option. Decisive actions like rapid encirclement such as performed on the large scale through Blitzkrieg and small scale like the Soviet "Deep Battle" called for were now possible like never before. Precision bombing and paratroopers also factors heavily into this.
Artillery. /u/jonewer explains it best here in that link. Artillery had nothing short of a renaissance during the war; he explains it very well in that link though so I'd just read that! :)
The lions share of fighting occurred in the East in WWII. Trench warfare, on a large scale that is, did not occur on the Eastern Front or really anywhere else in WWI for the same reason it did not here: large open spaces. Going from a mountainous very defensible 400 mile long front where only about half of that is even being seriously fought over to a practically entirely flat and open 2000 mile long front changes things quite a bit; static defensive lines are not at all viable in the latter case. You'll note that during D-Day though the highly defensive terrain, the bocages primarily, slowed the battle down to a crawl.
