The simplest reason why honey isn't vegan is by definition. The term vegan was coined by Donald Watson in 1944 and was defined as follows:
Veganism is a way of living which excludes all forms
of exploitation of, and cruelty to, the animal kingdom, and includes a
reverence for life. It applies to the practice of living on the
products of the plant kingdom to the exclusion of flesh, fish, fowl,
eggs, honey, animal milk and its derivatives, and encourages the use of
alternatives for all commodities derived wholly or in part from animals
