Using examples from the Arab world, how can I explain why and how certain dishes have come to be seen as belonging to ‘national’ cuisines?
"National" cuisines usually begin their development in response to the items that are easily obtainable for use as food in a given area. The foods that are most often eaten, because they are the most available, become a natural starting point for the invention of new ways of preparing dishes that add variety to the same basic ingredient(s).
Given that the "Arab world" encompasses a wide range of geographic areas, there are many different dishes that could be considered as part of one of the traditional Arabic cuisines. On the Arabic Peninsula, the traditionally plentiful food items would include barley, dates, meat (lamb and chicken most often), rice, and wheat. Dairy influence would often come from use of labneh, a yogurt without butterfat.
For breakfast, labneh might be served along with sfiha - an open-faced meat sandwich made with ground mutton and wheat flour. The main meal of the day, eaten after noon prayers, might include salad and maraq - a tomato sauce made with meat and vegetables - served over rice. The evening meal traditionally was much lighter.