Marx and Engels believe that the victory of the proletariat over the bourgeoisie is inevitable for two related reasons. First, they say that the capitalist system makes it impossible for the proletariat to continue to exist. Second, they say that capitalism forces workers to come together and unite, thus ensuring that they will defeat the bourgeoisie.
First, the capitalist system makes it impossible for the proletariat to exist on its own without any help. Marx and Engels say that the system creates a situation in which the worker
instead of rising with the progress of industry, sinks deeper and deeper below the conditions of existence of his own class.
This is not sustainable and will inevitably weaken capitalism.
Relatedly, capitalism forces workers to associate with one another, and thereby form a united front that will one day defeat capitalism. Because capitalism exploits the proletariat so badly, it forces them to join together to try to resist. They are no longer split up as individuals or small groups. Instead, they eventually join together and come to understand how badly they are being exploited. Since there are so many more workers than there are bourgeoisie, it is inevitable that a united working class will be able to overcome the bourgeoisie. Therefore, the victory of the proletariat will inevitably occur.