@santosh : Iron Man survival was a trade off for Time Stone
Also, Loki had traded the Tesseract for the promise that he’ll allow Thor to live.
But, did or can Thanos honor his promise while snapping is questionable ? Also, he snapped while losing the battle to Thor with his Strombreaker penetrating him ....so did he have the time to decide who to protect
It could be completely a chance / luck the original Avengers survived and that would be a infinitely low chance or maybe the survival of original 6 made a better story for the writers / director. However , scripting in Avengers is very tight and maybe the reveal why these 6 survived might come in final movie.