Why did Europe—particularly since 1757, when the British began their conquest of India at the Battle of Plassey—and also the United States in their brief 1898-1901 foray into the Philippines, ultimately win all colonial wars for nearly two centuries, despite a few lost battles?

We could simply say that they had better weapons. This would be a suitable answer if the West had lost its material superiority since then, which is far from being the case.
In the colonial or postcolonial context, wars are asymmetric, a conflict between the strong and the weak. They are irregular wars, involving guerrilla warfare and/or terrorism, in contrast with conventional wars, which are waged between two opponents who have agreed to fight a frontal battle, agreed to clash, theoretically with substantially equal forces.

For Europeans, the first colonial-type war—even if it was not in its time perceived as such—was Spain’s conquest of Mexico, which preceded all the others, from the conquest of Peru to those in Asia and in Africa in the eighteenth and nineteenth centuries and through to the mid-twentieth century.