Currently is is simply a convention. It could have been done both ways. The important thing is that everyone agrees so as not reverse the bit order.
To find why it was originally done one would have to find the first uart or maybe even the first use of serialized data. Not sure how many decades back and what level of technology one would have to look into.
Heck it could go back to semaphore flag signaling, and at that level of technology it would also be strictly a convention too.