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There are over 200 Upanishads but the traditional number is 108. Of them, only 10 are the principal Upanishads: Isha, Kena, Katha, Prashan, Mundaka, Mandukya, Tattiriya, Aitareya, Chhandogya and Brihadaranyaka. This book is a forerunner in introducing these primary Upanishads to the uninitiated. (Source) .
So what makes a major upanishad a major upanishad, what are qualifications and criteria for a major upanishad.
This classification must have originated from the way the Upanishads' "importance" are described in the Muktika Upanishad (linked to Shukla Yajurveda), which lists the names of the 108 Upanishads.
Hanuman: How many are the Vedas and how many branches do they have ? Of these what are the Upanishads ?
Rama: Vedas are four, Rig-Veda etc., many branches and Upanishads exist in them. Rig-Veda has 21 branches and Yajus has 109. Sama has 1000 and Atharva has 50. Each branch has one Upanishad. Even by reading one verse of them with devotion, one gets the status of union with me, hard to get even by sages.
I-i-15-17. Hanuman: Rama, sages speak differently: some say there is only one kind of liberation. Others say it can be got by worshipping your name and by the Taraka mantra at Kashi. Others speak of Sankhya-Yoga and Bhakti-Yoga, the enquiry into Vedanta-Vakyas etc.
I-i-18-23. Rama: Liberation is of four kinds: Salokya etc. But the only real type is Kaivalya. Anybody even though leading a wicked life, attains Salokya, not other worlds, by worshipping my name. Dying in the sacred Brahmanala in Kashi, he will get the Taraka-mantra and also liberation, without rebirth. On dying anywhere (else) in Kashi, Maheshvara will utter the Taraka-mantra in his right ear. He gets Sarupya with me as his sins are washed away.
The same is called Salokya and Sarupya. Persevering in good conduct, with mind fixed upon me, loving me as the Self of all, the twice-born gets nearer to me – This is called the three forms of liberation. Salokya, Sarupya and Samipya.
I-i-24-25. Meditating on my eternal form as prescribed by the Teacher, one will surely achieve identity with me like the insects changing into the bee. This alone is the liberation of identity (Sayujya) yielding the bliss of Brahman.
All these four kinds of Mukti will be got by worshipping Me.
I-i-26-29. But by what means is the Kaivalya kind of Moksha got ? The Mandukya is enough; if knowledge is not got from it, then study the Ten Upanishads. Getting knowledge very soon, you will reach my abode. If certainty is not got even then, study the 32 Upanishads and stop. If desiring Moksha without the body, read the 108 Upanishads. Hear their order.
I-i-30-39. 1. Isa 2. Kena 3. Katha 4. Prasna 5. Munda 6. Mandukya 7. Taittiri 8. Aitareya 9. Chandogya 10. Brihadaranyaka
Brahma
Kaivalya
Jabala
Svetasva
As you can see, according to Muktika, the highest knowledge is obtained from Mandukya alone. If that is not possible, then recommendation is to study the (first) ten (in the list given). From this people have probably concluded that those 10 are the principal/major (Mukhya) Upanishads and the rest are Gaunya or minor. However, in reality most people consider not just 10 but 12/13 Upanishads as major.
The major Upanishads are those that were commented on By Sankaracharya.
Sankaracharya wrote his celebrated commentaries on the Isa, Kena, Prasna, Mundaka, Mandukya, Aitareya, Taittiriya, Chhandogya, Brihadaranyaka, and possibly also the Svetasvatara Upanishad. These are regarded as the major works.
The Upanishads, General Introduction, by Swami Nikhilananda
Since Sankaracharya wrote commentaries on eleven Upanishads, he must have done so because he felt that these are the most important Upanishads.
Why are some Upanishads considered to be "major" while others are not considered to be so?
This is so only because of some traditional and historical reasons, namely because of how things have developed throughout history in the Hindu tradition, and not because Shankara thought that some Upanishads are major, nor because Muktika Upanishad supposedly says that some of the Upanishads are major.
If you learn about the history of Vedanta from books such as A History of Early Vedanta Philosophy by Hajime Nakamura you will see that throughout the history some Upanishads established to be the "major" ones by prominent Vedantists of ancient times, and that is because in their discussions of Vedanta they put more focus of their attention only on 10-15 Upanishads while the rest of Upanishads where not in their focus of attention. However, it is not entirely clear why did ancient Vedantists of those ancient times were just like that put more focus of their attention only on 10-15 Upanishads. And we know that Shankara was not the first Vedantist to think like that, and we also know that these old Vedantists did not quote from Muktika Upanishad.
So, why are some Upanishads considered to be "major" while others are not considered to be so is not due to Shankara alone, nor because of Muktika Upanishad, but is more, like, due to some traditional and historical reasons which are not entirely clear to us today.
