Here is why the Kingdom of Italy and Mussolini's "Roman Empire" failed:
1. Back in Roman times they were the largest and only unified state in all of Europe. All Empires who could surpass or match Rome were thousands of miles away from Italy. 19th and 20th century Italy had to deal with the powerhouses of France, Britain, Germany, Russia, United States etc.
2. Rome was the top industrious nation in ancient times and saved several technological innovations of the greeks. During Italy's time they were almost entirely agrarian with no natural resources to build the foundation of a heavy industry on. During the 19th century all of the leading scientists/inventors in the world mostly resided in Germany, Britain, United States.
3. Unlike Rome who had a large amount of political unity until the 5th century, Italy was filled with political infighting due to the conflicting needs of monarchists, nationalists, republicans, socialists, Catholics, etc.
4. Back then Rome could simply conquer any area with no consequence, Italy couldn't expand without facing the wrath of the Great Powers.
5. In Rome Latin was the dominant culture and all conquered peoples were forced to assimilate into it. At the time of Unification, Italy had so many different cultures and dialects that there really wasn't even a common Italian culture to exist in the first place. It took decades for these people to come to the idea of the Italian culture or even share a common language. In my opinion they should have never unified in the first place.
6. With advanced organization and weaponry Rome was the premier military power of its time. Italy's military just plain sucked so bad that they lost to Ethiopia. ETHIOPIA!! They had shit generals *cough* CADORNA *cough*, no advanced military tactics or technology, they made the stupidest battle plans, the nation had so many socialists that a good number of the population was always opposed to its wars. Granted modern military is decent but they are far from best in the world.
7. Every single one of their conquests made no sense and was just invaded for prestige. Oh yeah lets launch an invasion of Greece even though the country has no valuable resources, no Italian peoples, has a poor economy, and will only suck up troops when we're trying to conquer Egypt. 10 OUT OF 5 WORST IDEA EVER!