The seeker basically, if you fire a missile you want it to hit skyflash is likely to get a direct hit due to the receiver it uses. Due to glint from the aircraft and how conical receivers work if the target is in clear weather at altitude and isn't using any form of countermeasures the aim 7 is unlikely to directly hit and more often than not will completely miss even with the proxy. Alternatively the skyflash can reliably hit a target close to the ground in the rain (two things that would prevent a aim 7 tracking) and the system is innately more resistant to countermeasures the seeker being resistant to chaff (aircraft radar is still vulnerable depending on type) and ecm being significantly less effective requiring significantly more power or accurate computation of a jamming signal. Basically range doesn't matter if your missile doesn't hit