Possession of SSN's makes everyone else's surface fleets a target - the threat is well known, proven (see "Belgrano") - and almost impossible to defend against. It really crimps people's options.
No -one knows if AIP and other modern, non-nuclear, boats are as effective, or effective at all. If Australia thinks it may need to deter China they'll want keep them as far away as possible and the conventional boats can't get very far north - pretty much limited to E Indonesia and SSW Pacific. The UK has shallow water areas in the N Sea but a large area of the N Atlantic to cover as well. A single SSN can cover a far larger area than a conventional boat and you have much greater flexibility - albeit at an eye watering cost