I think KB is at least partly right. Oz is at the end of the line from Europe.
The only reason for SQ to want to get into bed with QF is for the trans-Pacific routes. After all, they can fly from SIN to anywhere in Oz and Europe that QF can. Any link up with SQ would probably result in QF feeding SQ's SIN hub to the detriment of the onward services - remind me where do QF fly to in Europe vs SQ's nework?
As for CX, pretty similar except they don't need the trans Pacific routes as they already have them. Direct HKG flights from Oz likewise already there with the European connections.
As for IB, then I don't think there are any direct flights at all to Spain, indicating a lack of demand.
Because of the above, a link up with BA allows the trunk Kangaroo route further expansion and for both carriers to utilise the extensive geographical networks that they have in their own areas. OK, you could argue that this is bascially already covered under the current agreements, and I wouldn't argue, but the industry has to consolidate and there are savings to be had. As suggested above, I think BA are QF's best fit option and probably (other than a QF - ANZ tie up) the smartest game in town.