There are a couple of reasons why this is the case, but the main difference is that Ripple does not have any way to "unconfirm" a transaction. Once a validator has signed a validation that includes the execution of a transaction, it cannot revoke this validation. So the moment anyone sees sufficient validations for the same ledger that includes a transaction, they know that that transaction is irrevocably confirmed.

With Bitcoin, you don't know who the miners are. So you have no easy way of knowing which blocks will ultimately survive in the chain with the most proof of work. So you have to wait until that chain is so much longer than any competing chain that a longer chain not including it is unlikely.

Another difference is that bitcoin only produces a block every ten minutes. This time choice is partly just historical accident but it's also limited by tradeoffs inherent in the network. Ripple's consensus isn't competitive, so there's no need to have long block times to prevent slow nodes from being at a disadvantage. Ripple's consensus algorithm can run almost as fast as the network is capable of carrying its messages.

In short form: Bitcoin has ten minute block times and requires 3 confirmations or so for high assurance. Ripple has five second ledger times and requires only a single confirmation or so for high assurance.