The instruments or forces of production are indeed important but they are not determining. For example, in order for the transition from feudalism to capitalism to occur it was essential that the forces of production had developed to a given level. For example, it was necessary that the agricultural instruments and techniques had developed to such an extent that they allowed people to be engaged in producitve activities other than agricultural production. Yet there is a difference between saying that the development of the forces of production is essential or necessary and saying that they are the dynamo of history. It is men, real, living men, who do this, who possess things and fight battles.
History is nothing but the activity of men in pursuit of their ends.
History is thus human activity, or to be more precise, the class struggle is the source of social change. The ends that people pursue are their class interests, those things that are beneficial to them as a class. Of course the material conditions must first exist. You must have the steam engine if you are to have the proletariat, and thereby also, the capitalist class. Yet the arrival of such inventions does no in itself produce historical change.