What happened to the earlier superpowers?

Roman Empire

It was probably the first empire that can be called a superpower. It was founded in 27 BC and lasted five centuries (also the longest superpower). Rome ultimately fell due to internal factors like civil war and economic depredations.

Modern historians also posit factors including the effectiveness and size of the Roman Empire army, the health and numbers of the Roman population, the strength of the economy, the competence of the emperors, the internal struggles for power, the religious changes of the period, and the efficiency of the civil administration. Increasing pressure from invading barbarians and many epidemic diseases also contributed greatly to the collapse.

Mongol Empire

It was the world's largest land empire. Just a million Mongols conquered vastly larger populations and empires. The Silk Road was the economic backbone of the Mongols. From 1206 till about 1294, Genghis Khan and his heirs ruled an empire that included most of Eurasia, much of the Middle East, parts of Eastern Europe, China and Russia. 

But ultimately, the empire began to fragment as the central government in China weakened. Although most historians consider Kublai Khan an able ruler, his empire was so large that even Kublai had trouble effectively governing all of it. After Kublai Khan's death the empire quickly dissolved.

Ottoman Empire

Ottoman was a state that controlled much of South-eastern Europe, Western Asia, and Northern Africa between the 14th and early 20th centuries. Under the reign of Suleiman, the Magnificent, the Ottoman Empire marked the peak of its power and prosperity as well as the highest development of its government, social, and economic systems. With Constantinople (modern-day Istanbul) as its capital and control of lands around the Mediterranean Basin, the Ottoman Empire was at the centre of interactions between the Eastern and Western worlds for six centuries.

At the start of World War I, the Ottoman Empire was already in decline. The Ottoman army entered the war in 1914 on the side of the Central Powers (including Germany and Austria-Hungary) and were defeated in October 1918. The end of the empire was marked by the abolition of the Ottoman Sultanate and the creation of the secular Republic of Turkey.

Great Britain

Before the United States and the Soviet Union appeared in world politics as superpowers, imperial Great Britain was rightfully a superpower. It had exercised colonial domination over 25% of the global population residing on 25% territory of the world. Britain's decline from the top of the world system was triggered by various reasons. 

Fighting two World Wars within a relatively short period had weakened Britain's economic strength and resources. In the post WW2 period, The USA and the Soviet Union emerged as the stakeholders of the bipolar world order pushing Britain to the brink.

Soviet Union

The collapse of the Soviet Union was the greatest event of 1990s world politics. The fall of the world's largest country was bewildering but not surprising. Many events contributed to the dissolution of this superpower. 

President Mikhail Gorbachev's reformist policies of glasnost and perestroika paved the initial roads to the collapse. These dual policies to render open discussions and a quasi-free-market created unrest all over the Eastern Bloc. By the end of 1989, the Berlin wall had fallen and the iron curtain was dismantled.

The Soviet's failure in Afghanistan was a key point behind the USSR's decline. The decade-long stay in Afghanistan wasn't quite beneficial for the country. Rather, it cost them a huge military expenditure amid a slowly failing financial system. They finally pulled out of Afghanistan in 1989 resulting in unrest in the region. 

All Baltic nations declared independence during this tenure. The collapse of the USSR was only a matter of time then.