He most likely wasn’t expecting to see you.
He probably said “Hi” and proceeded to completely ignore you because he was taken by surprise and felt awkward.
There is a good chance that he is fully aware that you are beautiful and considered as such by many guys. This perception may actually be somewhat intimidating to him.
There are many guys who can get intimidated by a woman’s beauty.
This may be why he gave you such a quick smile and immediately returned to looking at his phone and listening to his music.
He left when the bus stopped because he had most likely reached his stop and trying to acknowledge you in any way at that moment would have just made him feel even more awkward.