Why no Expedia AMS-LHR-SYD VAA availability

I've been looking at AMS-LHR-LAX/JFK options for UCS to the US and expedia offer various VAA fares from AMS that are significantly cheaper than the equivalent VAA LHR-JFK or LHR-LAX fares for example.
However, when I come to look for the same AMS origin fares to Sydney, there doesn't appear to be that option - anyone know why [?]? I'd love to go via AMS (and back!) if it meant a similar reduction as the US fares offered.[}:)]
Thanks.
However, when I come to look for the same AMS origin fares to Sydney, there doesn't appear to be that option - anyone know why [?]? I'd love to go via AMS (and back!) if it meant a similar reduction as the US fares offered.[}:)]
Thanks.