Why does mold grow on silicone caulk?
This product brochure seems to give a reasonable explanation:


  The following conditions help to form mould on sealants:
  
  
  High air humidity with little movement of air, eg: in bathrooms, showers, kitchens etc.
  Warmth
  Nutrients in the form of organic deposits. e.g. residues from body care materials
  such as soap, shower gel etc.
  
  
  Since elastic silicone joints have low thermal conductivity they are the warmest part of a tiled surface and where mould thrives best, in combination with organic residues and moisture.