To put it simply, terrain.

The Italian/Austrian Border is almost entirely mountainous, consisting of the Dolomites and the Eastern Alps Mountains which stretches down to near the Adriatic Sea. Mountain terrain greatly favors the defender in a war as the attacker has to make extra arrangements to transport equipment such as artillery and munitions.

In most cases, the Alps were nearly impassable for large concentrated armies with the narrow and harsh terrain, sending a column of over 100,000 soldiers through a narrow mountain pass is simply asking for disaster, should the defender want to use explosives to trigger a landslide (which Austria-Hungary and Italy both did on many occasions throughout the war), or simply shell them with artillery while they're all packed in a narrow area with no room to maneuver.

The last major obstacle is simply the high altitude of the mountains as well, where the decreased oxygen makes fighting and marching considerably more exhausting, with increased chances for frostbite even in the summer months where snow still falls and temperatures are hovering around freezing even in mid-July near the top of the Dolomites.

Instead, Italy and Austria-Hungary both employed much smaller and more specialized infantry units to maximize mobility in mountain terrain such as the Italian Arditi units, but the mountains themselves meant that neither army was going to accomplish much along the northern border.

This isn't to say that no fighting took place along the Alps Mountains, the Battles of Monte Grappa involved over 250,000 soldiers on both sides and was fought at an elevation of 1775 meters (5,800+ feet), making one of the highest elevated battles in history.

But with the challenges of mountain warfare, the grand majority of military attention was focused on the Isonzo River Valley, a narrow bottleneck of flat land between the Alps Mountains and the Adriatic Sea near the Austro-Hungarian city of Trieste. This valley is where the bulk of the fighting took place, with neither side able to break the other's position until the Battle of Caporetto in November of 1917, where Austria Hungary managed to break in and drive into the edges of the Po River valley in Italy, where the Italians launched a counterattack in 1918 and broke the Austro-Hungarian Army.

Primary Source:

Page, Thomas Nelson, (1920) "Italy and the World War". New York, Charles Scribner's Sons

Chapter 15 describes the significance of the mountain passes in more detail