Historical and political reasons too. USA keeps arming Pakistan with modern weapons, and they have a single target to use them on. It would be stupid of India to put all it's weight behind US if that's a one-sided affair. In an ideal world everyone would side with what is moral, unfortunately we don't live in that world.

More than old friends. India is heavily reliant on russian arms. Replacing them would be a slow, expensive process. Considering it is surrounded on two sides by aggressors with regular conflicts, jeopardizing relations with Russia is not an option.