Botulinum is a sporing bacteria, like anthrax; in conditions it can't reproduce well in, it forms spores, basically dormant versions of the bacteria inside a protective shell that can then withstand environmental conditions, including honey.

Honey is mildly antibacterial because it has such a strong concentration of sugar that it forces all the water out of bacteria by osmosis. Botulinum in a spore can survive that and then germinate into the active bacteria if it later enters safe conditions.

The other reason honey is specifically a risk is bioaccumulation. Botulinum spores naturally occur at low levels in most soil, which means there are tiny tiny traces on most things, including the surfaces of flower—not enough to do anything most of the time. But honey is made with of a lot of pollen. It can potentially build up in the honey the same way that eg. mercury builds up in tuna; tiny fishes absorb a little environmental mercury, but tuna eat a lot of tiny fishes in their lifetime, and they can't eliminate the mercury, so they consume far more mercury than a fish the same size would absorb from the water.