Shale formations differ from sandstone in that porosity exists as natural fractures that could run any number of directions. In sandstone it's more homogenous over the entire formation. Permeability is the measurement of the connectivity of the porosity. If you have porosity and those pores are filled with oil, but no connectivity between those pores you have no oil movement in the formation. Oil can not move to the well bore. Shales have less permeability than sandstone in general, where porosity is present. So, vertical penetration of shales is not conducive to commercial producton because the natural fractures occuring in shales are not consistent. And a vertical entry does not intersect the fractures at right angles, or much at all. However, when you turn and go sideways numerous fractures can be drilled through and fracing extends these natural fractures and creates new ones for the oil to migrate to the wellbore.
Before horizontal drilling, even the most prolific shale formations were teasers. During vertical drilling youd go through a good shale formation and you would show a good drilling break, there would be oil shows on the mudtanks, and the gas sniffers would go nuts, then the water would swell the naturally occuring clays in the shale and pretty soon the formation face would be sealed tight and during drill stem flowtests would give up nothing.