Why do Ismailis Pray Dua instead of Namaz like many other Muslims?

The prayer-form that most Muslims use today - called Namaz or Salah - was not taught by the Prophet Muhammad himself; it was actually derived and interpreted by the Sunni jurists during the first 200 years after the Prophet and validated by the Sunni legal tool of ijma or "consensus of the scholars." Whatever prayer-form the Prophet Muhammad actually taught and used is not known today and scholars can only find a few minute details about it from the Qur'an. For example, historians know that the Prophet's prayer-form included recitation of some Qur'an verses and it included bowing and prostrating. The Qur'an also states that the Pagan Arabs before Prophet Muhammad already prayed in the same way as Prophet Muhammad's community, except that the Prophet did some of the same actions in a different order - this is reported in the earliest Tafsir of Muqatil b. Sulayman on Qur'an 96:19 which talks about how the Pagans used to perform sujud, qiyam and ruku' in a different sequence from the Prophet Muhammad. All of this is based on historical analysis using the Quran as a historical document.

The procedures of what is today called Namaz or Salah as practiced by most Sunni Muslims are the result of the Sunni Jurists and Ulama interpreting the Qur'an and Sunni Hadith corpus. Even then, a large number of Muslims such as certain Sufi groups in South Asia, the esoteric Shia groups like the Ahl-i Haqq and Alawis and Alevis in the Middle East, and other Muslim groups do not practice the Namaz as the mandatory prayer-form. The Ismaili Muslims follow a living Imam in direct lineal descent from Prophet Muhammad and Imam 'Ali b. Abi Talib because the Ismaili Imam is the rightful bearer of the authority of Prophet Muhammad, so it matters little how the rest of the Muslim Ummah prays; nor does it matter precisely how the Prophet Muhammad and his Community prayed. The Ismaili Imam prescribes and authorizes the best and most correct method of prayer in every time-period of history.

This model of continuous Divine Guidance reinterpreted in every time is the same model of Divine Guidance used by Prophet Muhammad when he recited the Qur'an to the Community and gave guidance. The Prophetic-Qur'anic Guidance model is a model of CONTINUOUS, PROGRESSIVE, and RESPONSIVE guidance - that means, whatever the situation of the early Community, both the Qur'anic guidance recited by the Prophet and the Prophet's own guidance on all matters including ritual practices would CHANGE and UPDATE and EVOLVE as new situations arose and circumstances changed - and this is an undeniable historical fact proven from the Qur'an itself:

The number of prayers changed from 2 times in Makkah to 3 times in Madinah;
The rules on drinking alcohol changed at least 3-4 times;
The guidance for warfare changed several times as the circumstances changed;
The practices of fasting - how to fast and when to fast - changed at least 2-3 times.

The Quran and Prophet were changing and updating the Divine Guidance to the Community in an ongoing manner as times and circumstances changed - and all this happened only over 23 years. The Prophet Muhammad KNEW that Divine Guidance would have to continue after his death - that is why he said that the Kitab of God (meaning Divine Guidance) and his Ahl al-Bayt would never separate from one another and told the Community to follow Imam Ali as the Mawla (Master) because the Imams continue giving Divine Guidance to the Community that is also CONTINUOUS, PROGRESSIVE, and RESPONSIVE. The Qur'an even says that the Imams in the periods before Prophet Muhammad had the Divine authority to establish the form of prayer for the Believers:

And We made them Imams guiding by Our command. And We inspired to them the doing of good deeds, to establish the prayer, and giving of zakah; and they were worshippers of Us
- Qur’an 21:73

“By the institution of the Ulu'l-Amr - who can be interpreted as Imam and Caliph - and by placing obedience to the ulu'l-Amr immediately after that to God and Prophet, he ensured that the Faith would ever remain living, extending, developiing with science, knowledge, art, and industry... If, rightly, the Muslims have kept till now to the forms of prayer and fasting at the time of the Prophet, it should not be forgotten that it is not the forms of prayer and fasting that have been commanded, but the facts, and we are entitled to adjust the forms to the facts of life as circumstances changed. It is the same Prophet who advises his followers ever to remain Ibnu’l-Waqt (i.e. children of the time and period in which they were on earth), and it must be the natural ambition of every Muslim to practice and represent his Faith according to the standard of the Waqt or space-time.”
– Imam Sultan Muhammad Shah Aga Khan III,
(Foreword to Muhammad: A Mercy To all the Nations by Al-Hajji Qassim Jairazbhoy, Read Here)

Today Mawlana Hazar Imam had made the prayer-form of Dua and not Namaz/Salah to be the obligatory prayer-form for Ismaili Muslims, because this is the Divine Guidance that has been ordained for the Believers in the present day and age. It is true that in the past, the Ismailis or some Ismailis prayed the Namaz as the main prayer but in the era of Imam Sultan Muhammad Shah, all of this was changed and the Jamats of Syria, Iran, and Central Asia all were commanded to recite the Ismaili Du'a as the mandatory prayer-form three times per day.

The fact is, the Namaz as practiced by most Sunni Muslims and the Dua of the Ismaili Tariqah are not only different in their gestures, actions, and content, but these two prayer forms are not even used in the same context. The Namaz belongs to the category of Shariah, it is unform in terms of the gestures not the recited content, and it can be done by and in front of anyone. The Ismaili Dua is part of the Tariqah category ( read about what is a Tariqah here), it is uniform in its recited content, it is done 3 not 5 times per day, and it is a private prayer when it is practiced. When one deeply examines the Namaz/Salah and the Ismaili Du'a, one will find that the content of the Ismaili Dua is the ta'wil or esoteric dimension of the gestures of the Namaz/Salah, as explained here: https://ismailignosis.com/2013/03/03/the-esoterics-batin-of-prayer-from-salah-to-dua/