(Of course biological weapons are bad, but I don't get this preference of yours)
Maybe in some ideal scifi world.
In this one “biological weapons” are just disesases modified and packaged for warfighting. Their efficiency is measured in how little of the substance you need to kill or disable someone, and also in how reliably it kills/disables.
A headshot kills you instantly. From biolgocial weapons the ones which kill you (because do note, that is not always the aim) will torture you for hours but more likely for days before you succumb. Even longer if you get medical help. That is of course if you are lucky enough and the attack hasn’t overwhelmed the medical system where you are. Which of course they are designed to do.
Biological weapons can unleash hell on earth. They are difficult to aim with and they do not discriminate between combatants and non-combatants.
Where did you get the notion that they “hurt less”?
Biological weapons can mutate or spread in unforeseen ways to infect victims other than initial targets, and a virus or bacterium does not destinguish between combatants and non-combatants. A biological weapon can cause suffering on a much larger scale than the initial deployment of the weapon, which is why they are widely banned.