Why can't water soluble hormones diffuse across a lipid bilayer?

The lipid bilayer is hydrophobic (insoluble with water...think of it as being made of oil or butter), which means only other hydrophobic molecules can pass through it. Hydrophilic (water-soluble) molecules can't get through the hydrophobic center of the bilayer because it is just solid enough that there is no space between the lipid molecules, and the hydrophilic molecules can't interact with the hydrophobic molecules enough to make them soluble with the bilayer and diffuse through.