I have no inside information, but my interpretation has been that the location of the SkyTeam-branded lounges is focused on airports that:

• Are not an existing hub of an ST airline (otherwise that airline would normally provide lounge facilities for the alliance); and:
• Are served by a reasonable number of ST airlines/flights (to justify the investment and the shared branding); and:
• Are important locations where ST wants to anchor its presence in the face of the competition (eg: LHR – BA, plus London being an important travel hub anyway; IST – TK; DXB – EK; I’m not so sure about SYD, but I guess Australia’s a white-spot for ST, and SYD is served by at least 7 ST members with long-haul flights)

There are presumably also factors such as the current provision of lounge facilities at any given airport (and hence the opportunities for cost-savings, etc), as well as the availability of a suitable space in which to put a SkyTeam lounge.