Zia ul-Haq was a symptom of Pakistan's problems, not a cause. I guess my question is, why has there never been a military coup in India and why does the military exercise so much more power and influence in Pakistan than it does in India? Why are Pakistan's democratic institutions so much weaker than India's even though they both started off in more or less the same position in 1947?
That's incredible easy to explain.
India and Pakistan are each others arch enemies.
India have 10 times the population of Pakistan (5 times the population, when Bangladesh was East Pakistan).
This mean that Pakistan need to invest a lot of resources and money into the army, while India need to invest less. That make a military coup easier in Pakistan than India, simply because there are more soldier per capita in Pakistan and the officer corps are a greater part of civil servants in Pakistan than India.
These greater investmentd in the army are also one of the main reasons that Pakistan do worse than India in general, as India can invest more in the civil society.