It’s a timeless, elegant classic with lots of nicknames.
Cass,
Cassie,
Sandy,
Andy,
Andra,
Candy, etc.
Cassidy seems more juvenile, it’s part of the surname trend that is already sounding dated and needs to die, and is a lot more limited for nickname potential. Unless you count “Butch
Cassidy” which she might get if she’s not particularly feminine (which is totally fine, but kids are cruel).