Why is this mantra called a “mahamantra”? Is it described so only by Karshnavs/Vaishnavs?
A well known Hindu scholar Stephen Knapp in his book Yoga and Meditation has provided an answer to your question why this mantra is called as Mahamantra. There is a separate chapter called Hare Krishna Mantra in the book.
In a section of that chapter - The importance of Mahamantra the author is explaining about the Hare-Krishna-Hare-krishna- HareRama-Hare-Hare mantra.Below are the excerpts.
All mantras and all processes for self-realization are compressed into the Hare krishna Maha mantra.
These sixteen words - Hare-Krishna-Hare-krishna ,krishna krishna Hare Hare / Hare- Rama , Hare- Rama Rama Rama Hare Hare - are especially meant for counteracting the ill effects of the present age of quarrel and anxiety-
The reason that chanting the lord's names is such an effective process is because lord and his names are identical : they are the same spiritual energy . By chanting Hare Krishna we are in immediate contact with god in proportion to the chanter's purity.
He also is giving refrences about this mantra from Shreemad Bhagvatam and Chaitanya Charitamrita in the section ,citing its importance.
कृते यद्ध्यायतो विष्णुं त्रेतायां यजतो मखै: ।
द्वापरे परिचर्यायां कलौ तद्धरिकीर्तनात् ॥ 52 ॥
kṛte yad dhyāyato viṣṇuṁ tretāyāṁ yajato makhaiḥ
dvāpare paricaryāyāṁ kalau tad dhari-kīrtanāt
Whatever result was obtained in Satya-yuga by meditating on Viṣṇu, in Tretā-yuga by performing sacrifices, and in Dvāpara-yuga by serving the Lord’s lotus feet can be obtained in Kali-yuga simply by chanting the Hare Kṛṣṇa mahā-mantra. SB 12.3.52
Hare krishna maha-mantra is said to be the sound incarnation of krishna and anyone who chants this mahamantra is in direct association with krishna and is delivered from the clutches of the material energy or Maya. (Adi-lila 17.22)
So these are some of the reasons why it is considered as maha mantra by Vaishnavas.
In summery -
- This mantra is effective espicially in this age of kali as it reduces ill effects of the present age i.e. quarrel and anxiety .
- All mantras and all processes for self-realization are compressed into the this mantra.
- By chanting Hare Krishna we are in immediate contact with god .
There are other Maha mantras also like Om Namo Narayana and Om Vishnave na Maha etc. , but mainly AFAIK this mantra is considered as Maha Mantra by some Vaishnava Sects like Gaudiya Vaishnavas (Iskcon) and Varkari sect of Maharashtra.