Why does Ripple have a minimum balance?
The Ripple asset is unique in that unlike other assets Ripple enforces reserve requirements in Ripple (XRP) to protect the shared global ledger from growing excessively large as the result of spam or malicious usage.
From the Official Ripple Documentation…
The goal is to constrain the growth of the ledger to match improvements in technology so that a current commodity-level machine can always fit the current ledger in RAM and the full ledger history on disk.
To submit transactions, an address must hold a minimum amount of XRP in the shared global ledger. You cannot send this XRP to other addresses. To fund a new address, you must send enough XRP to meet the reserve requirement.
The current minimum reserve requirement is 20 XRP. (This is the cost of an address that owns no other objects in the ledger.)
You can read more information on the Ripple reserve requirements in the official documentation.