Surplus to requirements. It was old , kicked out of the pack, or flock, or whatever. Alternative was letting it wander until wild dogs ate it alive. Nature red in tooth and claw, the twitters disparaging Rebecca Francis don’t know that.
My understanding is that older male giraffes maintain a harem of female giraffes that they use for breeding, even after age catches up with the older male, and he is "shooting blanks" and no longer reproducing. In other words, the older male giraffe is still boinking his harem, but he ain't making any babies. When a hunter or predator takes out an old male giraffe, a young male giraffe in the prime of his reproductive life takes over the harem to produce a baby boom. I also understand that hunting fees are used to fund the conservation efforts and that the hunters are required to provide the meat from the giraffes to the locals.