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.
A ban usually means that the account is closed forever, while a suspension is temporary.
But even if it’s not, brands like to distinguish themselves by using different lingo to their competitors. For example, even though Lemmy is a Reddit clone is heavily inspired by Reddit, they use different words like community instead of sublemmy.
A ban would imply that the user cannot come back, but they make money off of users data, so they WANT you to come back, no matter how bad your behaviour was.