Ayesha Jalal: When Pakistan was created, it got a financial structure that was 17.5% of undivided India, and a military that was one-third of undivided India. [In other words, the areas that made up Pakistan contributed just 17.5% of India’s tax revenue before 1947 and inherited about one-third of undivided India’s military.] Pakistan could never square that [gap].
On top of it, with Kashmir and all the problems with India, the Army emerged dominant because it was able to hook up with the international capitalist system, America in particular. They also got on to various security alliances [formed because of Cold War politics post-World War II] that tipped the balance against politicians. More here.