Why are gustatory receptors sensitive to dissolved chemicals but insensitive to light?

Gustatory receptor cells are located within the taste buds found on the papillae of the tongue.  When tastants dissolve in the saliva, they can be "tasted" by triggering the release of neurotransmitters which cause a message to be sent to the brain through sensory neurons.  When salty food are eaten, they liberate Na+ ions which enter the gustatory receptor cells via sodium channels, causing depolarization, and neurotransmitter release.  Sour foods liberate H+ ions which enter the cells through proton channels.  The other tastes (sweet, bitter, and umami) activate G-coupled protein receptors and activate second messengers inside of the receptor cells to trigger neurotransmitter release.  

The cells of the eye, however, are sensitive to light rather than tastants.  Within the rod cells found in the retina there are discs covered in molecules of rhodopsin.  Rhodopsin is a photopigment consisting of two parts:  opsin and retinal (derived from vitamin A--this is why they say that carrots are good for your vision).  When light is absent, the retinal as a cis conformation and fits into the opsin.  However, when light is present, the cis-retinal absorbs the light and isomerizes to trans-retinal.  Trans-retinal has a straight shape rather than a bent shape and no longer fits into the opsin molecule.  It dissociates from the opsin.  

How is this related to signals sent to the brain indicating the presence of light?  In the absence of light, when cis-retinal is present, Na+ flows into the photoreceptors through ligand-gated sodium channels.  Cyclic GMP is the molecule that opens these channels.  The influx of sodium makes the cell less negative (-70mV-->-30mV) and partially depolarizes the cell.  This partial depolarization triggers neurotransmitter release.  However, the neurotransmitter is glutamate.  Glutamate is an inhibitory neurotransmitter and sends inhibitory messages to the next cells in line (bipolar cells) thus preventing messages from being sent to the subsequent cell layer (ganglion) cells and out through the optic nerve to the brain.

When light is present and trans-retinal forms, enzymes that break down cyclic GMP are activated.  When cyclic GMP is broken down, it can no longer open the ligand-gated sodium channels so the sodium cannot enter the cell to partially depolarize it.  This turns off the release of the glutamate neurotransmitter and stops the inhibitory message from being sent to the bipolar cells.  They can then send messages to the ganglion cells which can carry the message to the brain via the optic nerve indicating the presence of light.

So the reason that gustatory cells are not sensitive to light is that they do not contain photopigments like rhodopsin that are sensitive to light.  The triggering of gustatory receptor cells is due to tastants and the taste neurons that they activate.