Why did Sri Krishna say "I am the VAJRA among all weapons"?

The following Prologue would be necessary for presenting my view on this topic.

I will present my perception of the sacred texts coming into existence.

a ) Vedic Literature  -  No one can exactly fix the period at which they came into existence.

b) Srimad Ramayana - An embodiment of Vedic Dictums told along the Sri Rama's story. -  May be 1.6 million years old

c) Mahabharata - Some Indian scholars fixed the age of Mahabharata war as 3139 B.C.  I prefer to follow those scholars to believe the Western scholars and Indian scholars with Western bent of mind.

d) Srimad Bhagavad Gita - A part of Mahabharata and told to Arjuna around 3139 B.C.

Till this period Vishnu, Shiva were not elevated to the status of Supreme Gods.  We do not even find Ganapati in Bhagavadgita, though he was mentioned in Puranas as Gannadhipati.  His existence might have come up in Puranic period.

e) Puranas - written eulogising the greatness of respective dieties. viz., Bhagavatam - Vishnu;  Shiva Purana - Shiva, etc.  Age of Puranas is very recent compared to Vedic Literature

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The status of Devatas as mentioned in Vedic Literature, i.e., Brahma, Indra, Agni, Varuna, Vishnu, Aryama, Kaartikeya, etc, continued in Srimad Ramayana period and upto Mahabharata Era also.  

1)       अनन्तश्चास्मि नागानां वरुणो यादसामहम्।
पितृ़णामर्यमा चास्मि यमः संयमतामहम्।।10.29।।

 Of the snakes, I am Ananta; of the water-beings (water-deities), I am varuna; of the manes, I am Aryaman; of the controllers, I am Yama (the Death-god).

2)        आदित्यानामहं विष्णुर्ज्योतिषां रविरंशुमान्।
मरीचिर्मरुतामस्मि नक्षत्राणामहं शशी।।10.21।।

Of the sons of Aditi, I am Visnu; of the luminaries, the radiant Sun; of the Maruts, I am Marici; of the stars, I am the Moon.

3)         Indra was the Chief of Gods.  This status continued upto Mahabharata period.  That is why he was mentioned as so even in Bhagavad Gita also.

वेदानां सामवेदोऽस्मि देवानामस्मि वासवः।
इन्द्रियाणां मनश्चास्मि भूतानामस्मि चेतना।।10.22।।

“Of the Vedas, I am the Samaveda; of the gods, I am Vasava (Indra); of the sense-organs, I am the mind; of the beings, I am the sentience.”

4)         Vishnu was the younger brother of Indra (Indraanujum).  He is one among all the Devatas only.  

5)         Rudra was present at that time, but Shiva  was not found Vedic Pantheon.

6)         Kaartikeya was the Senapati of Devatas.  His status continued as it was.

पुरोधसां च मुख्यं मां विद्धि पार्थ बृहस्पतिम्।
सेनानीनामहं स्कन्दः सरसामस्मि सागरः।।10.24।।

And, among the household priests (of kings), O Arjuna, know Me to be the chief, Brihaspati; among the army generals I am Skanda; among lakes I am the ocean. '

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7.  Indra's Vijra was the most powerful weapon.   Hence, this status continued upto Mahabharata period.  That is why he was mentioned as so even in Bhagavad Gita also.

आयुधानामहं वज्रं धेनूनामस्मि कामधुक्।
प्रजनश्चास्मि कन्दर्पः सर्पाणामस्मि वासुकिः।।10.28।।

Of the weapons, I am the Vajra [of Indra]; of the cows, I am the Wish-fullfilling Cow [of the heaven]; of the progenitors, I am Kandarpa (the god-of-love); of the serpents, I am Vasuki.

8)  We will come across a mention about VISHNU CHAKRA in Srimad Ramayana.  While searching for Seeta, Sri Hanuma saw Ravana, who was sleeping.  

ऐरावत विषाण अग्रैर् आपीडित क्र्त व्रणौ |
वज्र उल्लिखित पीन अंसौ विष्णु चक्र परिक्षितौ ||  (Sundara Kanda 10^th Sarga 16^th Sloka)

“Those arms had wounds made by stabbing from the pointed edges of tusks of Iraavata and fleshy shoulders smeared with diamond and hit by Vishnu chakra.”

If VISHNU CHAKRA , i.e, Sudarshana, is so powerful as claimed in the Puranas, Ravana should not live further.  He would have been dead then there itself, but he survived.

9)   The TRISHULA of Shiva found place from Puranas only.  That is why it did not find place in Srimad Bhagavad Gita, which preceded Puranas.

10) Hence, Sri Krishna said that he was VAJRA among all weapons.