Why did K. Marx, F. Engels and their supporters consider the proletariat to be the main force for transforming society?

Why did K. Marx, F. Engels and their supporters consider the proletariat to be the main force for transforming society? Where did they see the advantages of the proletariat over other classes and strata of society?

Marx, Engels and their supporters considered the proletariat to be the main force in transforming society because, according to Marxism, with the development of capitalism, the position of the working class would inevitably worsen, which would lead to an aggravation of social contradictions.
Marx, Engels and their supporters saw the advantages of the proletariat over other classes and strata of society in its numbers and progressivity.

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