Why did Tsongkhapa list guru devotion first in his Lamrim while Atisha did not? Lama Atisha famously brought the teachings of Nalanda back to Tibet and authored his famous, A Lamp for the Path to Enlightenment upon which the new translation schools were founded. This seminal text is the foundational document of the lamrim upon which all subsequent lamrims are based.

Atisha did not seem to mention guru devotion in his lamrim. Spiritual teachers are mentioned only in the course of explaining how to properly receive the Bodhisattva vows and to receive Tantric initiations. Nowhere can I find the idea that guru devotion is the root of the path to enlightenment.

Je Tsongkhapa on the other hand listed guru devotion towards the very beginning of his lamrim and it is widely taught in new translation schools that guru devotion is the root of the path to enlightenment. Why is this? Why would something so important and fundamental be left out entirely by Lama Atisha from his seminal text?

Also, this is what His Holiness the Dalai Lama has said about guru devotion in Questioning the Advice of the Guru:


  It is frequently said that the essence of the training in guru yoga is
  to cultivate the art of seeing everything the guru does as perfect.
  Personally I myself do not like this to be taken too far. Often we see
  written in the scriptures, “Every action seen as perfect.” However,
  this phrase must be seen in the light of Buddha Shakyamuni’s own
  words: “Accept my teachings only after examining them as an analyst
  buys gold. Accept nothing out of mere faith in me.” The problem with
  the practice of seeing everything the guru does as perfect is that it
  very easily turns to poison for both the guru and the disciple.
  Therefore, whenever I teach this practice, I always advocate that the
  tradition of “every action seen as perfect” not be stressed. Should
  the guru manifest unDharmic qualities or give teachings contradicting
  Dharma, the instruction on seeing the spiritual master as perfect must
  give way to reason and Dharma wisdom.
  
  Perhaps you will think: “The Dalai Lama has not read the Lam Rim
  scriptures. He does not know that there is no practice of Dharma
  without the guru.” I am not being disrespectful of the Lam Rim
  teachings. A student of the spiritual path should rely upon a teacher
  and should meditate on that teacher’s kindness and good qualities; but
  the teaching on seeing his or her actions as perfect can only be
  applied within the context of the Dharma as a whole and the rational
  approach to knowledge that it advocates. As the teachings on seeing
  the guru’s actions as perfect is borrowed from Highest Tantra and
  appears in the Lam Rim mainly to prepare the trainee for tantric
  practice, beginners must treat it with caution. As for spiritual
  teachers, if they misrepresent this precept of guru yoga in order to
  take advantage of naive disciples, their actions are like pouring
  the liquid fires of hell directly into their stomachs.


Emphasis mine. If preparing the trainee for Tantric practice is the reason for adding it to the Lamrim, then is it true that guru devotion is not of such importance for those studying sutra only? In other words, can it be said that for those practicing sutrayana only that guru devotion is not the root of the path?
Very good question. This is a quote by Lama Zopa Rinpoche that seems to cover this point.


  Lama Tsongkhapa asked Manjushri, “What is the way to achieve quick
  realizations of the path to enlightenment?” Manjushri answered, “To
  purify the mind of the obstacles to realizations and to collect the
  necessary conditions, the merit, to collect extensive merit, that’s
  one thing. The other one is one-pointedly making requests to the
  guru,” that means with devotion. The third one is meditating on the
  path.


Thus it may have been under Manjushri's guidance that Je Tsongkhapa adopted this order.

You are of course right about the order in which the Lamrim points are presented is different in the various Lamrims. While Jowo Je Atisha used Bodhichitta as the first step on the path, Gampopa in his "Jewel Ornament of Liberation" begins with Buddha Nature. While Patrul Rinpoche begins "The Words of My Perfect Teacher" with the precious human life.

Atisha begins his Lamrim after explaining the three scopes with the following verse:

For these excellent beings
Who aspire to supreme enlightenment,
I shall explain the perfect method
Taught by my spiritual masters.

Thus the Lamp on the Path to Enlightenment is primarily addressed to Mahayana practitioners and thus generating the Spirit of Enlightenment is the first step he presents.

Gampopa has a very interesting presentation in "Jewel Ornament of Liberation". Buddha Nature is the primary cause, the precious human life is the working basis while reliance on a spiritual master is the contributory cause.

Patrul Rinpoche's combines 'Reliance on a Spiritual Master' together with generating the spirit of enlightenment and development of Bodhichitta. One of my teachers has explained that the above are three levels of refuge and Patrul Rinpoche uses it as the preliminaries before the introduction of Mahamudra, Chod and Transference teachings.

Since the Lamrim Chenmo is a sutric text it does not go into the tantra except towards the end where Je Tsongkhapa mentions that the disciple has to practice the teachings of the Vajrayana before undertaking the four methods of gathering disciples. However the Vajrayana is a part of the entire path and thus from Je Tsongkhapa's point of view the preliminaries should include guru devotion.

This is explained further in Panchen Chyoki Gyaltsen's 'Easy Path' where each section is divided into the preliminary, the actual meditation session and time in between sessions. This format is adopted by Kyabje Pabonka Rinpoche as well in 'Liberation in you Hands'. In the preliminary, is the primary practice of Guru Yoga and supplication which helps you understand the meditation topic better.

Thus in the Gelug Lamrims following Tsongkhapa the importance on the Reliance on the Spiritual Master being the root of the path is emphasised. This can be best explained by the benefits of relying upon a spiritual master as found in the Precious Master's Instructions Outline to the Lamrim.

The eight benefits of relying on spiritual masters:
* Coming closer to achieving Buddhahood
* Pleasing all conquerors
* Being invulnerable to demons and misleading friends
* The automatic reduction of disturbing mental factors and incorrect behaviour
* The increase of all grounds, paths and realisations
* Never being deprived of a teacher in all future lives
* Not falling into lower rebirths
* Effortlessly achieving all temporary and ultimate goals