Is there any reason or any sutta tells why Buddha choose sacred fig for meditation and austerity?
The ‘Asathu / fig’ Bodhi Tree (Ficus Religiosa) ඇසතු බෝ (අස්සත්ත) - was born, it is believed, on the same day as the Buddha Himself, who sat under that tree in his quest for Enlightenment. Also others such as Princess Yasodhara, Kanthaka Horse, his elephant, his charioteer Channa, Ven. Ananda etc. were born on the same day and year. The term Bodhi is used by Buddhists to denote any tree under which a Buddha has attained Enlightenment.
If you take the seven Buddha of the last 90 kalpa,.... Vipassi Buddha attained Enlightenment under a Palol tree (Stereospermum chelonoides) පලොල් (පාඨලී ); Sikhi Buddha under a Etamba tree or Sri Lankan Mango tree (Mangifera zeylanica) ඇටඹ (පුන්ඩරික); Vessabhu Buddha under a Maha-sala tree (Shorea Robusta) ඉන්දියානු සල් (සාල) ; Kakusandha Buddha under a Mara tree / Gulmohar tree or the Royal Poinciana (Delonix regia) මාර / මහරි / (සිරිස); Konagamana Buddha under a Attikka (Dimbul) Ficus glomerata. අට්ටික්කා (උදුම්බර); Kassapa Buddha under a Bayan tree (Nigrodha) නුග / මහනුග (නිග්රෝධ).
All Buddhas attain Buddhahood under a tree. Asathu Bodhi Tree was meant to be for Gauthama Buddha. That is why it sprung up on the day Bodhisatwa was born. Please note that trees which serve as Bodhi for all the Buddhas had sprung up on the days when those Bodhisatwas were born for the last time. A Pipila Tree had shaded Deepankara Buddha කප්පිත්ත / පතන බෝ of the past (whom many know of from the Suttas). It is said that Maithri Buddha’s Bo Tree will be a ‘Na’ or Iron Wood Tree.
One guess I have is that, according to the first sutta referenced in this answer, he chose the place (a piece of countryside) for other reasons (i.e. he chose the place because it had a river, villages, etc.). Maybe he chose the Bodhi tree because it was there.
Another quote is something I remember from this article: Oldest Buddhist Shrine Uncovered In Nepal May Push Back the Buddha's Birth Date
She was cautious about the oldest Buddhist shrine claim.
"The worship of trees, often at simple altars, was a ubiquitous feature of ancient Indian religions, and given the degree of overlap between Buddhist ritual and pre-existing traditions, it is also possible that what is being described represents an older tree shrine quite disconnected from the worship of the historical Buddha," Shaw says.
I assume that it's now called "sacred" and religiosa because the Buddha chose it, not vice versa.
Also you can see from images like this one that when it's big its roots provide shelter for sitting.