Ma nishtana is only an example of questions. When we ask questions, this is a proof that our attention is good. There are procedures in the Seder made to enhance our awareness. After a good starting, the attention remains good at the time of sipur yetsiat mitsrayim.
Gemara says in Pesachim 115b
אביי הוה יתיב קמיה דרבה חזא דקא מדלי תכא מקמיה אמר להו עדיין לא קא אכלינן אתו קא מעקרי תכא מיקמן אמר ליה רבה פטרתן מלומר מה נשתנה
Abaye did seen that they clear the table, and asked "he still not begins to eat and you already cleared the table? This question fulfilled the role of ma nishtana. So, to ask questions early enhance attention. To wait for the end make concentration difficult and the ceremony boresom.