I had read somewhere (a very reliable source) that USSR had warned India against prolonging the war on the western front. USSR warned that such action would lead to USSR withdrawing it's support to India. USSR alongside the US put pressure on India for early release of the 90,000 war prisoners so that India could not use them as bargaining chip during negotiation.
Independence of Bangladesh would have created an opportunity for the USSR to gain a base in the Bay of Bengal, read the history of
BAKSAL. However it could not be realized due to assassination of Mujib and maneuvering by India.
India had her sweet revenge from USSR for it's dubious role in 1971, when she refused to open another front to put pressure on Pakistan during Soviet-Afghan war.