Why do molecules, including H2 and more complex organic molecules, only form inside dark clouds? Why don't they fill all interstellar space?

A molecular cloud also called a stellar nursery, is a type of the interstellar cloud, with the composition of commonly the molecular hydrogen,

In dark clouds, the accumulation of the dust in a specific location, which blocks the starlight and hence, stopping ionization, and thereby making that area dark.

They do not fill all of the intersteller space, because, the number of dust grains only account for a fraction of the volume of the intersteller space.