From what I have heard he was offered a contract extension by Saints but decided against signing it. It's a very similar situation with Joe Burgess and Ryan Sutton at Wigan, and I guess there'll be a few others also awaiting the Aussie call.
The three of them I believe still have one year left on their contract so things may change, but the money on offer over there will be much higher. If Wigan and Saints chose to get near or match the offers then it puts them in a perilous position when the next really good youngster comes through.
Leeds had a similar problem with Jordan Baldwinson and Mason Tonks and they were allowed to leave to go to NZ Warriors (although both have returned after failing to make an impression in their U20's side)
edit - I hadn't heard of any offer for Savelio, just that he didn't want to sign his extension, so that would tie up with the OP