There were more than one reason for India's division...
(1) Poet Iqbal.
(2) Mohamed Ali Jinnah.
(3) Jawaharlal Nehru and Congress party.
(4) Religious fanaticism on both sides.
(5) Traditional "divide and rule" policy of England.
Last but not least is the main reason for the division. For those who ruled India for 90 years (1857-1947), it would have been easy for them to put down uprisings by the separatists, who, till date continue to be "separatists". But the English govt. encouraged Jinnah from behind and made him a hero just to fulfill their aspirations of colonising India again. For those who staged the Jallianwalah Bagh massacre in 1919, it would have been easier to deal will Jinnah & Co.
If India was not divided, it would now be the largest country in the world and would have overtaken China and USA in terms of economy and population. The people of Pakistan would now be in a better economic position and their talents would have been utilised for creative purposes.