Just a decision AFAIK. Both have up and downsides.

Example upsides of ():

• () is easier and faster to type with a standard keyboard layout (and in a shell you want to type fast)
• shell-users are used to it (bash uses $())
• {} is fairly common in things like json and thus would require more escaping

Example upsides of {}:

• It is less common in text written by humans for humans and thus you have to escape it less often
• new python users are more used to it (in python2 only %s existed)
• rust users are more used to it (tho rust and nu fulfill very different use-cases)