U.S. sanctions prevent Iran from updating its 30-year-old American aircraft and make it difficult to get European spare parts or planes as well. The country has come to rely on Russian aircraft, many of them Soviet-era planes that are harder to get parts for since the fall of the Soviet Union.
My info is that crew insited on landing despite below minima because of a sick pax on board . So this one has not much to do with lack of spares.(The first part of the above statement is however correct )In any case, plenty of Tu154 spare parts in Russia, as this A/c was Russian registered.
An Iranian friend of mine tells me that the preferred method of travel is bus.
If you knew how iranians drive and the yearly death doll on the roads in Iran you would prefer Air travel when you can.