What could be the reasons for reduced peripheral vision besides glaucoma?

Many. Strokes, retinal detachments, retinitis pigmentosa, other optic neuropathies, etc.
Several reasons. Peripheral vision can be decreased by any condition that either blocks side vision or decreases the transmission of images coming from the periphery. Examples include cataracts, corneal scars, drooping eyelids, retinal disease and nerve damage.