Why do we recite surahs in only the first two rakats while offering the farz prayer but in all four while offering the sunnah prayer?
The Prophet pbuh taught the prayer in this way that we read Surah Fatihah and a part of the Qur'an in the first two units and recite only surah fatihah in the last two units and do not recite the extra part of the Qur'an in these two units. Now the question is if it is obligatory to recite the qur'an over and above surah fatihah in the first two units? Will my prayer invalid if I omit the Quran reading over Surah Fatihah in the first two units? The answer is no. It is still valid. The obligatory part is only Surah Fatihah. Since Surah fatihah is the part of the Qur'an the obligation to recite the Qur'an is fulfilled by reading it alone.
Some jurists consider the qira'ah of the surah in the last two nafl payer rak'ats because they believe that every two units in optional prayer are independent units. They are so independent that if the later two units are invalidated let us say for example due to loss of wudu the first two would not be affected and will be considered duly offered.