its a question to me, Why while Iran is working and producing 100 mm Sa'ir, 35 mm Samavat, and 23 mm Mesbah-1, But dont upgrade these ZSU-57-2's or ZSU-23-4's with modern radar and electro-optical, laser tracking systems and new modern fire control system.
Maybe one of reasons is that they are very old. I think they are around 40 years old. but it seems they are still using them.