All of these examples were ultimately achieved by military conquests and are closely associated with legendary founders (who were also conquerors). The initial Indian unification was carried on by the Maurya, under Chandragupta, and later continued by his decedents, most famously Ashoka who, after the bloody conquest of Kalinga, converted to Buddhism and eschewed violence altogether. You can observe similar military figures in the history of Chinese unification under Qin Shi Huang, who arose after the dissolution of the Zhou Dynasty into a warring states period, forcefully overcame all of his rivals, and unified China in the Qin Dynasty.
Finally, in Southeast Asia and the Middle East, you have the example of Cyrus the Great, legendary founder of the Persian Empire. The stories of expansion and unification are closely tied with military campaigns, often carried out by a dominant warrior king who stands in the center of that story.
You can look at these examples through the contexts of the larger histories of the regions in question. If you were to look towards the Persian and Middle Eastern context, you'd be looking at a succession of large empires. Consider how the Persians were ultimately conquered by Alexander the Great. Alexander's empire broke up into the Hellenistic Successor States, which would give way to the Romans in the West and the Parthians (and later Sasanians) in the East, after which the entire region would be conquered by Islam.
On the other hand, consider the Indian context. The Maurya provided a period of unification, only to fragment and dissolve (unification was ultimately not lasting in this case). Later, large-scale empires in India would include the Guptas, and after the arrival of Islam, the Mughals. Finally, consider the Qin, which, it should be said, was not actually the first ruling Dynasty of China (the Qin themselves should be understood in the context of fragmentation of the earlier Zhou Dynasty) and which did not last long past the death of its founder. The Qin Dynasty was ultimately replaced by the Han Dynasty. This allowed the political unification achieved by the Qin to ultimately continue under a different ruling dynasty. The Han would last for over 400 years, after which China would again be broken up, only to be reunified again under the short-lived Sui (who would be followed by the much longer-lived Tang).
It might be useful to consider not only the ways that these larger contexts diverge from one another but also perhaps the ways they mirror each other.
The Indian subcontinent has been unified numerous times in recorded history, however the only time it has been fully unified have been under the rule of the British Raj (British colonial rule) and he current Indian republic ( excluding Pakistan and Bangladesh). The other empires to successfully control large portions of what is now know as India have been the Moghul Empire and The Mauryan Empire. The unification processes were very similar to those that occurred in China and the Near Eastern empires, for the primary reason being that all the unification processes involved some form of conquest or warfare. The unification processes were also similar, due to the fact that in each case the resulting empires or states were all united by similar cultures. They shared similar languages and religious beliefs (in the case of India, Hinduism or its derivatives) and had a shared political history.