Why Not Just Chant “Krishna”?
Caitanya Mahaprabhu is Krishna Himself. Therefore by following Lord Caitanya you are following God Himself, Lord Sri Krishna. That same Supreme Lord Sri Krishna who spoke the Bhagavad-gita to Arjuna 5,000 years ago has reappeared as Sri Caitanya Mahaprabhu a mere 500 years ago to teach us what is the most effective means of awakening our dormant Krishna consciousness by chanting the Hare Krishna mahamantra. Therefore, just as Krishna's instruction to chant His name is authoritative, Lord Caitanya's instruction to chant Krishna's name by chanting the Hare Krishna mantra is equally authoritative.
The effectiveness and pre-eminence of the Hare Krishna mantra in the present age of Kali is further confirmed in the Kali Santarana Upanishad as follows:
(The Kali-santarana Upanishad is one of the 108 principal Upanishads. In text 1 to 11, the maha-mantra is established as the only means of deliverance in Kali-yuga)
hari om. dvaparante narada brahmanam
jagama katham bhagavan gam
paryatana kalim santareyam iti. 
"Hari Om! After travelling all over the world, Srila Narada Muni approached Lord Brahma (his own father and guru) during the time when Dvapara-yuga was about to end. He asked him, “O Lord! How will all the living entities be able to deliver themselves from the most degraded age of Kali which is about to begin.”
sa hovaca. brahma sadhu prishto’smi
sarva shruti rahasyam gopyam tac
chrinu yena kali samsaram tarishyasi.
Lord Brahma replied, “You have asked the most important and relevant question for the benefit of humanity. I will tell you now the most concealed secret of the Vedic literatures, with the help of which everyone can easily cross this most dangerous age of Kali.
bhagavata adi purushasya narayanasya
namoccharana matrena nirdhta kalir bhavati 
“Simply by chanting the transcendental names of the original enjoyer and the Supreme Personality of Godhead, Lord Narayana, all the sins will be cleansed in Kali-yuga.”
narada punah papraccha tan nama kim iti.
Narada Muni again asked,“Which are those specific names of the Lord which are most effective in Kali-yuga?”
sa hovaca hiranyagarbah
hare krishna hare krishna, krishna krishna hare hare.
hare rama hare rama, rama rama hare hare; 
Lord Brahma replied, “Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare; Hare Rama, Hare Rama, Rama Rama, Hare Hare.
iti shodashakam namnam, kali-kalmasha-nashanam;
natah parataropayah, sarva-vedeshu drishyate. 
This sixteen words mantra are especially meant for completely destroying all the contamination of Kali. To save oneself from the contamination of Kali-yuga, there is no alternative in all theVedas except the chanting of this sixteen-word mantra.
(Some editions of Kali Santarana Upanishad describe the Hare Krishna Maha Mantra with Hare Rama first and then Hare Krishna. But such a change seems to have been made in recent editions of Kali Santarana Upanishad published by Ramanandis who want to put Lord Rama's Name before Lord Krishna's. As quoted, in the Ananta Samhita, Shri Chaitanya Upanishad (Atharva Veda), Agni Purana, Sanat-kumara Samhita and Vayu Purana, the Hare Krishna maha-mantra begins with Hare Krishna first.)
iti shodasha kalasya jivasyavarana vinashanam;
tatah prakashate para brahma medhapaye
ravi rashmi mandali veti. 
“This maha-mantra consisting of sixteen names of Krishna is the destroyer of the (16-layered) illusory covering of all the living entities. Only when the coverings are destroyed, Lord Krishna, the Supreme Personality of Godhead will appear in the heart of the living entities, just like the sun appears with full intensity when the clouds are removed.”
punar narada prapaccha bhagavan kasya vidhi-riti 
Srila Narada again inquired from Lord Brahma-“What are the procedures and rules for chanting this maha-mantra?”
tam hovaca nasya vidhi-riti
sarvada shuci-ashucira va pathan
brahmanam salokya samipya
sarupatam sayujyatam eti. 
Lord Brahma replied, “There are no rules and regulations to chant this maha-mantra. It should be chanted always irrespective of whether one is in a pure or impure condition.
yadasya shodashikasya sardha trikotir japati.
tada brahma hatyam tarati vira hatyam.
svarna steyat puto bhavati. pitrideva
manushyanam apakarat puto bhavati. 
“When one chants the 16 names of the maha-mantra 35 million times (sardha trikotir), he absolves himself of the sins of killing a brahmana or a kshatriya, stealing of gold or engaging in harmful activities towards one’s forefathers or other living entities.
(If one chants 16 rounds of the Hare Krishna maha-mantra, he will complete the chanting of 35 million maha-mantras in around 55.5 years. If he doubles his chanting i.e. 32 rounds daily, then he will take around 28 years. And if he can chant the ideal prescribed quota of chanting i.e. 64 rounds daily, he will take around 15 years to complete the great sacrifice of chanting the maha-mantra 35,000,000 times. If one chants 64 rounds of the maha-mantra for 15 years while strictly following the principles of bhakti-yoga in the association of pure devotees, then one is sure to see the Supreme Personality of Godhead face to face in this very life and ultimately achieve the highest abode of the Lord called Goloka Vrindavana. )
sarva dharma parityaga papat sadyah sucitamapnuyat. sadyo mucyate sadyo mucyate ity upanishat. 
“By chanting the maha-mantra, one becomes quickly purified from all sins accumulated due to rejecting all of one’s duties in our past lives. Thus the Hare Krishna maha-mantra, is the only means, the only means, the only means of thoroughly and immediately purifying oneself in Kali-yuga. Thus ends the Kali-santarana Upanishad.”