Why Krishna is God?
Krishna is the source of everything. This is explained in BG verse 10.8 and BG Verse 14.4
Bhagavad Gita verse 10:8
ahaṁ sarvasya prabhavo mattaḥ sarvaṁ pravartate
iti matvā bhajante māṁ budhā bhāva-samanvitāḥ
Translation by HDG AC Bhaktivedanta Swami Prabhupad
I am the source of all spiritual and material worlds. Everything emanates from Me. The wise who perfectly know this engage in My devotional service and worship Me with all their hearts.
Bhagavad Gita verse 14.4
sarva-yoniṣu kaunteya mūrtayaḥ sambhavanti yāḥ
tāsāṁ brahma mahad yonir ahaṁ bīja-pradaḥ pitā
Translation by HDG AC Bhaktivedanta Swami Prabhupad
It should be understood that all species of life, O son of Kuntī, are made possible by birth in this material nature, and that I am the seed-giving father.
Krishna is the sustainer of everyone and the prime eternal person as explained in BG Verse 2.12 and BG Verse 7.10 and BG Verse 2.12
Bhagavad Gita verse 7.10
bījaṁ māṁ sarva-bhūtānāṁ viddhi pārtha sanātanam
buddhir buddhimatām asmi tejas tejasvinām aham
Translation by HDG AC Bhaktivedanta Swami Prabhupad
O son of Pṛthā, know that I am the original seed of all existences, the intelligence of the intelligent, and the prowess of all powerful men.
Bhagavad Gita verse 2:12
na tv evāhaṁ jātu nāsaṁ na tvaṁ neme janādhipāḥ
na caiva na bhaviṣyāmaḥ sarve vayam ataḥ param
Translation by HDG AC Bhaktivedanta Swami Prabhupad
Never was there a time when I did not exist, nor you, nor all these kings; nor in the future shall any of us cease to be.
Krishna is alive in the hearts of all living entities. All living entities are eternal and Krishna is the maintainer of everyone in the spiritual and material worlds.
We are conscious of our bodies and Krishna is conscious of all bodies as explained in BG verse 13.23
Bhagavad Gita verse 13.23
upadraṣṭānumantā ca bhartā bhoktā maheśvaraḥ
paramātmeti cāpy ukto dehe ’smin puruṣaḥ paraḥ
Translation by HDG AC Bhaktivedanta Swami Prabhupad
Yet in this body there is another, a transcendental enjoyer, who is the Lord, the supreme proprietor, who exists as the overseer and permitter, and who is known as the Supersoul.
Krishna is independent and free from the influences of the modes of material nature. Explained in BG verse 7:12
Bhagavad Gita verse 7.12
ye caiva sāttvikā bhāvā rājasās tāmasāś ca ye
matta eveti tān viddhi na tv ahaṁ teṣu te mayi
Translation by HDG AC Bhaktivedanta Swami Prabhupad
Know that all states of being – be they of goodness, passion or ignorance – are manifested by My energy. I am, in one sense, everything, but I am independent. I am not under the modes of material nature, for they, on the contrary, are within Me.
Krishna is not affected by karma as explained in BG verse 3.22
Bhagavad Gita verse 3.22
na me pārthāsti kartavyaṁ triṣu lokeṣu kiñcana
nānavāptam avāptavyaṁ varta eva ca karmaṇi
Translation by HDG AC Bhaktivedanta Swami Prabhupad
O son of Pṛthā, there is no work prescribed for Me within all the three planetary systems. Nor am I in want of anything, nor have I a need to obtain anything – and yet I am engaged in prescribed duties.
Krishna possesses mutually contradictory qualities. He is bigger than the biggest and smaller than the smallest simultaneously. This is explained in BG verse 13.14, 13.15, 13.16 and 13.17
Bhagavad Gita verse 13.14
sarvataḥ pāṇi-pādaṁ tat sarvato ’kṣi-śiro-mukham
sarvataḥ śrutimal loke sarvam āvṛtya tiṣṭhati
Translation by HDG AC Bhaktivedanta Swami Prabhupad
Everywhere are His hands and legs, His eyes, heads and faces, and He has ears everywhere. In this way the Supersoul exists, pervading everything.
Bhagavad Gita verse 13.15
sarvendriya-guṇābhāsaṁ sarvendriya-vivarjitam
asaktaṁ sarva-bhṛc caiva nirguṇaṁ guṇa-bhoktṛ ca
Translation by HDG AC Bhaktivedanta Swami Prabhupad
The Supersoul is the original source of all senses, yet He is without senses. He is unattached, although He is the maintainer of all living beings. He transcends the modes of nature, and at the same time He is the master of all the modes of material nature.
Bhagavad Gita verse 13.16
bahir antaś ca bhūtānām acaraṁ caram eva ca
sūkṣmatvāt tad avijñeyaṁ dūra-sthaṁ cāntike ca tat
Translation by HDG AC Bhaktivedanta Swami Prabhupad
The Supreme Truth exists outside and inside of all living beings, the moving and the nonmoving. Because He is subtle, He is beyond the power of the material senses to see or to know. Although far, far away, He is also near to all.
Bhagavad Gita verse 13.17
avibhaktaṁ ca bhūteṣu vibhaktam iva ca sthitam
bhūta-bhartṛ ca taj jñeyaṁ grasiṣṇu prabhaviṣṇu ca
Translation by HDG AC Bhaktivedanta Swami Prabhupad
Although the Supersoul appears to be divided among all beings, He is never divided. He is situated as one. Although He is the maintainer of every living entity, it is to be understood that He devours and develops all.
Krishna posses the 6 opulence in full. He is the source of all beauty, all strength, all fame, all wealth, all power and all renunciation.