Babur left his nascent kingdom after just 4 short years of rule. Within this time he was able to annex a large portion of the northern belt under him i.e from whole of Punjab till some parts of Bihar. His Mewar campaign however fell midway as he soon passed away due to an illness.
With the kings passing away, the succession battles get pretty tricky and potentially bloody even as many want to wrest power during this chaos. Babur was aware of this eventuality so he prepared as much as possible to make this transition smooth without any loss of life or kin. He had 4 healthy sons i.e 4 legitimate heirs to the throne. There was Humayun-the eldest, Kamran, Askari and Hindal all sons from his one legitimate wife Maham.
Humayun clearly was his favorite and he wanted him to succeed his empire but to placate his other sons he ensured that he divided his empire between them and assign each of them the governorship of their respective province. For Eg Kamran was assigned the governorship of Kabul. Despite the governorship assigned they still had to acknowledge Humayun as their king and obey him. These were Babur’s final directives, thus Humayun came to be the 2nd emperor of the Mughal empire.
Humayun was a gentle king, supremely intelligent, had a keen interest in astronomy and he thoroughly believed in mysticism and the occult, so much so that this fascination would actually culminate into weird practices. When he came into power he immediately tasted military success by annexing Gujrat in his kitty. Gujrat being a wealthy and prosperous port was a big win for him and his imperial treasury. But this was his one and only conquest, he would soon end up losing his entire empire and become exiled king for the next 30 odd years.
Where Babur was driven, aggressive and hungry to expand this empire, Humayun on the other hand was terribly lazy, content and relaxed. This doesn’t mean he was not capable, there is plenty proof of his military capabilities during his fathers campaigns. His only problem was he just was not interested in war making. He would much rather spend his time in his harem, with his wine and opium so much so that his vices soon started to take a strong hold over him. He became a drug addict.
Not only his drug problems, but he was also faced with a lot of opposition from his brothers most notably Kamran, who wanted to become king. He repeatedly defied his rule, turned down any military support and refused to contribute his earnings from the land towards the imperial treasury. He even launched (with the support of his brothers) several attacks against Humayun. Despite all this, Humayun was benevolent, he just didn’t take any action against his troublesome brothers- Kamran specifically- even when he stooped to such unimaginable levels of kidnapping his son (the future Akbar) and putting him in danger.
This was just one part of the problem. Even as a ruler, a general and an administrator- on all 3 counts he turned out to be hopeless. This is owing mainly to his drug addiction problems. So absorbed was he in wine and opium, he refused to attend court and take decisions. In fact he even started coming with absurd indicts like dividing the administration into Earth (for agriculture), Fire (for military), Water (for canals and waterways) and Air (for everything else) which has no practical or tangible implication. Coupled with this he failed to deal with internal dissent from his brothers with a strong hand, nor was he able to deal with the external ones from Afghans,Pustuns etc all of whom were vying for the lucrative throne of Hindustan.
And as it was bound to happen one clever and smart Pustun- Sher Shah Sur- ursurped his beloved kingdom from under his nose, pushed Humayun completely from Hindustan and successfully established the Sur dynasty. Humayun became king without a throne.