Here are a few things.

1) If someone happens to get the password for one of the services, it is nice if that does not mean they have the password for all of the services.

2) If a service account has been compromised and is misused, having it only associated with one service reduces the search area to find out what has allowed the breach and possibly who has made the breach.

3) Regular password maintenance will only impact a single service rather than every service.