A lot is about the temporality of the word. I would like to add the psychology as well. It is to lessen the blow when confronted with the news your account got banned. Using the words ‘suspended indefinitely’ for it has a different impact:
Your account got banned.
Your account has been suspended indefinitely until further notice.
It is the same reason why we like to say someone has passed away, instead of died. Or we like to use lobbying for bribery.