Did it have something to do with the air power/armor of both sides?
Trench warfare on the scale of the Western Front in WWI certainly wasn't the norm. The Italian theatre was quasi-trench/mountain warfare, but the major campaigns after Caporetto in October 1917 were quite mobile, as was the fighting around Salonika. Trenches at Gallipoli were imposed by logistical and terrain constraints, while trenches on the Eastern Front and in the Middle East were largely temporary cover for in between offensives, which were quite mobile. As for the Western Front, the trench stalemate only set-in in December 1914 really, and by 1917 it was already beginning to give way.
In the case of WWII, there was in fact positional warfare/trench warfare. The fighting on the Weygand and Maginot Lines was positional in June 1940, as it was against the Italians on the Alpine Line. Positional Warfare reigned in Northwestern Russia from the end of 1941 up until 1944, and there were periods of positional fighting in Italy from 1943-45, especially at Monte Cassino. The first two months of the Normandy campaign were positional, and casualty rates were higher than at Passchendaele in 1917.
What create the trench stalemate in 1914 was that both armies had even strength in men and weapons, but it was far more difficult to attack than defend, something that Clausewitz had acknowledged. Weapons systems greatly improved in and before WWII, so if anything the early 'lightning campaigns' of the Germans were the exception to the rule set by WWI. So long as Great Powers possessed the manpower, firepower, industry and morale to sustain the fighting, modern warfare would be very bloody and would involve long, drawn out struggles.
The crucial difference lies in motorization, and to a lesser extent improved wireless communications - in WWI, a breakthrough of the enemy line would effectively turn into a race between the attackers, having to march into the breach (going beyond the reach of their mainly wired comms and the extent of their rail network), supplied almost exclusively by horse, and the defenders rushing to block the gap in their line, usually via train, and on territory where they still had full access to their comms.
Naturally, in this race, the people riding trains tended to beat those marching on foot - and hence a huge advantage of maneuver for the defender, who also enjoyed greater cohesion and clarity of purpose due to the possession of a functioning wire network.
In regions not so well-developed as Western Europe the fighting tended to be much more fluid (and where offensives tended to enjoy more success overall), as these advantages to the defender diminished.
The decisive weapon of WWII was the automobile - not tanks, but the humble truck (for without trucks to hump their gas and ammo, tanks were relatively hobbled). With the advent of fully motorized units, the little successes which in WWI frequently petered-out after a few kilometers could be turned into scything penetrations stretching over hundreds of kilometers, with a minimal diminution of fighting power as the mighty truck tirelessly hauled supplies and reinforcements at speeds comparable to a train, to places the tracks could not reach. Units on the march could be guided and coordinated with relative ease by radio, instead of suffering the inertia caused by having to wait for runners or wire to be laid behind the advancing soldiers.
WWII saw, above all else, a revolution in logistics, allowing an attacker to grasp a significant advantage over a defender by taking the initiative - offensives could move with such speed that attempting to react to the enemy's actions often proved extremely difficult, as by the time the command of the defence could assess the situation and take counter-measures, the attackers made defensive measures taken redundant through the pace of their assault (i.e. the defence could move reserves to x position, but by the time they had done so the attacker could have traveled a very long way indeed - indeed, slow reactions through antiquated command & control structures frequently proved fatal). Tanks and aircraft were nice, but when both sides had some kind of parity, could not be truly decisive.
Temporary trenches were still very common. In "800 Days on the Eastern Front: A Russian Soldier Remembers World War II" they detail the very intricate trenches used by the Soviets for the Battle of Kursk. However, due to artillery, air, and especially armor improvements these trenches would slow the enemy down, but not stop them.
In the later half of the war, the Germans were constantly using trenches to slow down the Soviet advance. However, the combination of massive artillery barrages, air raids, and tank divisions would lead to gaps in these trenches.
So, trenches were used and they did significantly slow down the enemy. However, because the lines were constantly changing these trenches were often less developed than those seen in WWI.