AC did and may still codeshare LX to India (BOM?) And they may codeshare on the North Atlantic out of YYZ and YUL. Codesharing involves commitments. Typically, on international movements, the host carrier on whose metal the passenger travels will make available capacity and certain agreed prices to the codeshare partner. The codeshare partner may decide not to take capacity on some/many routes so as not to spread itself thin. AC, I presume, wants to funnel as much business as possible to partners through LHR and FRA and even MUC because it serves those airports from multiple places in Canada. If LX does something unique that LH doesn't, that would make sense for AC to codeshare, but it would not necessarily make sense to commit to LX capacity to Moscow, for example, if it has adequate capacity available on LH or SK.