Why Vaikuntha was not mentioned in Bhagavad Gita though Krishna was considered as the incarnation of Vishnu?

Vaikuntha was not mentioned in Bhagavad Gita.

Neither Rig Veda nor Brahmanas/Upanishads mention Vishnu's abode as Vaikuntha.

Rig Veda 1.154.6, dedicated to Vishnu (an epithet of the formless BRAHMAN), mentions Paramam Padam, which indicates SUPREME BLISS in spiritual connotation, but Vaishnavaites at a later date interpreted it as SUPREME ABODE of Vishnu.

Vaishnavaites might have adopted that name from Rig Veda 10.48, which mentions its seer as Indra Vaikuṇṭha.

 

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Though dhāma paramaṁ was mentioned in Bhagavad Gita 8.21, it should be understood in Advaita's concept of attaining SUPREME BLISS, as Shri Krishna delivered his words while remaining in the highest Advaitic State, but not as Vishnu's Vaikuntha.

अव्यक्तोऽक्षर इत्युक्तस्तमाहुः परमां गतिम्।

यं प्राप्य न निवर्तन्ते तद्धाम परमं मम।।8.21।।

avyakto ’kṣhara ityuktas tam āhuḥ paramāṁ gatim
yaṁ prāpya na nivartante tad dhāma paramaṁ mama

——

Rig Veda 10.181.2 mentions about dhāma paramaṃ, which was hidden, indicating it as SUPREME BLISS attained at the culmination of spiritual practices.

अविन्दन्ते अतिहितं यदासीद्यज्ञस्य धाम परमं गुहा यत् । धातुर्द्युतानात्सवितुश्च विष्णोर्भरद्वाजो बृहदा चक्रे अग्नेः ॥
avindan te atihitaṃ yad āsīd yajñasya dhāma paramaṃ guhā yat | dhātur dyutānāt savituś ca viṣṇor bharadvājo bṛhad ā cakre agneḥ ||

—-

Further, Rig Veda 1.154.6, dedicated to Viṣṇu also mentions about Paramam padam, indicating the Supreme BLISS that can be attained by a spiritual practitioner.

ता वां वास्तून्युश्मसि गमध्यै यत्र गावो भूरिशृङ्गा अयासः । अत्राह तदुरुगायस्य वृष्णः परमं पदमव भाति भूरि ॥
tā vāṃ vāstūny uśmasi gamadhyai yatra gāvo bhūriśṛṅgā ayāsaḥ | atrāha tad urugāyasya vṛṣṇaḥ paramam padam ava bhāti bhūri ||

——

However, Vaishnava traditions (like those of Ramanuja or Madhva) interpret the Gita's dhāma paramaṁ very differently, viewing it as a direct reference to Vaikuntha.

We have to remember that it is in the later Puranas that the distinct, localized abodes for the Supreme deities become clearly defined: Vaikuntha for Vishnu, Kailasha for Shiva, and Manidweepa for Devi.