I'm not sure but I think the problem is that the data isn't just encrypted with the user's passcode, it's encrypted with the passcode entangled with the private key of the secure enclave chip (which, presumably, is unknown to anyone).
So they can't just attempt to brute force the encrypted data, as the encryption key would be 256 bits or more rather than just a 4 or 6-digit numeric passcode. That's why they want Apple to open the OS up to brute force passcode attempts.
Edit: there's no secure enclave chip on the phone in question, but it seems that iOS 8 and later encrypt the user data with a separate private key combined with the user passcode.
So they can't just attempt to brute force the encrypted data, as the encryption key would be 256 bits or more rather than just a 4 or 6-digit numeric passcode. That's why they want Apple to open the OS up to brute force passcode attempts.
Edit: there's no secure enclave chip on the phone in question, but it seems that iOS 8 and later encrypt the user data with a separate private key combined with the user passcode.