Why does the Vishnu Purana say that even gods praise being born in Bhārata varṣa?

 It is true that ambiance play a vital role in achieving many things. And, spirituality is no exception to this.

Ambiance is a totality of attitude of the people surrounding us, way of earning livelihood, way of eating habits, etc.

Bharata Varsha aka India does contain that ambiance for practicing spirituality, compared to other places. It does not mean to say ambiance cannot be created in other places too and people practicing in other countries will not be spiritually elevated.

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The shloka from Vishnu Purana 2.3.24 is as follows:

गायन्ति देवाः किल गीतकानि धन्यास्तु ते भारतभूमिभागे ।
स्वर्गापवर्गास्पद मार्गभूते भवन्ति भूयः पुरुषाः सुरत्वात् ॥

The gods themselves exclaim, “Happy are those who are born, even from the condition of gods, as men in Bhārata-varṣa, as that is the way to the pleasures of Paradise, or the greater blessing of final liberation.”

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Puranas were composed at a much later date to Rig Veda.

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There is an interesting reference from Rig Veda, wherein rishi prays for the blessings of Indra on the descendants of bhārata.

Rig Veda 3.53.12

य इमे रोदसी उभे अहमिन्द्रमतुष्टवम् । विश्वामित्रस्य रक्षति ब्रह्मेदं भारतं जनम् ॥

ya ime rodasī ubhe aham indram atuṣṭavam | viśvāmitrasya rakṣati brahmedam bhārataṃ janam ||

English translation:

I, who have praised both these two world-halves here, with Viśvāmitra’s stotra — and (may) Indra guards the Bharata people.

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In sanskrit bhā (भा) indicates "shine" or "be luminous". rata (रत) Translates to being devoted, engaged, or reveling in something. bhārata indicates spiritually inclined or engaged person.

Hence, descendants of bhārata indicates the people who are engaged or inclined in spiritual practices.

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The composers of Vishnu Purana might have taken the literal meaning of bhārata, and stated that People living Bhārata-varṣa, as mentioned above, without taking into consideration of spiritual meaning of Rig Veda.