One of the reasons is that while the Italian front did divert some (axis) men and material from other fronts, that is pretty much all it was good for.

Stalin (just like most people) thought that there was simply no way the allies could hope to move from Italy into France, or through Austria into Germany.

The Alps make for a very good defensive line, just look up how the Austrians continuously repelled the Italians from invading during the first world war, or how the French absolutely mauled the Italians when Mussolini wanted to carve a slice out of France during the blitzkrieg earlier in the second world war.

In order to properly threaten, and being able to invade Germany itself, the Allies simply must invade France, or maybe liberate Norway and Denmark, and go down the Jutland peninsula. France was obviously the better option, since it was much closer to the United Kingdom.

Partly, Stalin was a hard bastard for whom nothing was, diplomatically speaking, enough.

The real issue was the Italian Front was overall a bit of a mistake. It tied down only a small number of German divisions. Indeed, after the Battle of Sicily the Italian mainland was more or less devoid of large Wehrmacht units, they had to be rushed in when the Italians overthrew Mussolini and joined the Allies.

This link shows the levels of deployment, even in mid-1944 before D-Day there were three times more divisions preparing to repel the landings, than there were in Italy.

Italy was highly defensible, it was mountainous with multiple rivers running across the Allied lines of advance. The Germans diverted rivers to flood river valleys and render them impassible. Armour could not maneuver, aircraft struggled to find targets. The central mountain spine made it hard to transfer units from east to west.

The six-month Battle of Monte Cassino was the emblematic campaign battle, a hard and bloody slog. The Anzio landing was mismanaged, and conflicts between Allied commanders exacerbated problems.

Another major issue was that even if Italy had been over-run, it didn't pressure Germany very much. The Alps were a massive barrier to any break out.

Churchill sponsored a series of landings on various Greek Islands in 1944, which were an embarrassing failure as German paratroops recaptured the islands with support from Stuka dive bombers (in 1944!).

Only a landing in France would really pressure Germany to fight and commit resources, as Allied breakthrough was possible and would be catastrophic, and lead all the way into Germany itself. But the landing itself was an extremely difficult challenge- something Stalin always ignored.