Pickles have some kind of acids which generated naturally, due to which pickle's taste is sour. If it is stored in a metallic container,naturally generated acids will react with the metal & thus the compounds of metal can cause harm to health.Commercially prepared pickles have citric acid added as preservative & in some cases Acetic acid also . So due to presence of these acids these are not stored in metallic container.
Pickles are destroyed due to fungal effects in the long term , this fungal effect is minimised by sun light or natural lights,So storage them in a glass container, is for the easy approach of sun light to the pickle. You have also observed that after 3-4 month interval these containers are also placed for whole day in sun light for the same purpose. While storing pickles in metallic container , blocks the sun light approach thus due to fungus pickles damage possibilities are much more in short period.
Pickles are often stored for a very long time and other than glass all other materials get corrode due to the oil, acids and spices which are added to pickle. Glass is an inert material, it does not get affected by these substances.
Sour things are acidic in nature. Acidic materials react with aluminium and forms different compounds which may be harmful. So it is not advisable to store sour things in aluminium jars.
1 3 1