I think that communist theorists left it up to the bourgeois intelligentsia for two possible reasons.
First, they did so because the proletariat was not sufficiently educated. In a capitalist system, they would have said, the proletariat is not given enough education. Therefore, it is not at all likely that a communist movement would be able to find enough members of the proletariat who have the intellectual training necessary to create a full-fledged philosophy.
Second, they did so because the proletariat had been more thoroughly mystified by the bourgeoisie. According to communist theory, the owners of the means of production try various methods to keep control over the proletariat. They use religion. They exploit racial differences. They do things like this to prevent the proletariat from seeing that all workers have common interests and should join together to overthrow the ruling class. This means that the average member of the proletariat has had the wool pulled over his or her eyes and cannot really see truth without help. The members of the bourgeois intelligentsia have had education that can potentially liberate the mind from the ruses allowing those of the bourgeoisie the potential (not the guarantee) of therefore seeing more clearly.
These are the main reasons why it would be necessary to have the bourgeois intelligentsia help the proletariat in this effort.