The main point of conflict between Pakistan and India was Kashmir, that coupled with tensions between Afghanistan kept an overwhelming majority of our military in West Pakistan. There was a mutual understanding that both sides would not escalate any Kashmir-related conflicted to Bengal; this is highlighted in the 1947 and 1965 wars of which East Pakistan was relatively untouched from.
There was a some troop influx, but it was too little and too late. You must understand the logistical cost and time needed to transport a large amount of troops, here is a map just for a perspective.
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In order to properly fight India conventionally in East Pakistan, heavy assets such as artillery, tanks, fighter jets and others had to be transported along this route; which would have been an extremely difficult, risky and costly task. When war broke out, Pakistan only had one squadron in East Pakistan and very little heavy weaponry; Pakistan had absolutely no chance as brilliantly summarized by the Indian Commander of the 1971 war: