The big problems with bioweapons are deployment and control.
On the one hand, the biological agents involved can only live in a certain range of conditions, so your delivery system(s) have to not breach those while still spreading their contents over a wide area. It's much harder to design an ICBM warhead that delivers it's germ payload alive and over a usefully large area than to make one deliver a nuclear bomb intact, for example. Cruise missiles or aircraft are much better for that, but have their own problems with respect to delay and the possibility of interception. Another aspect of this is that there's usually a delay between the agent being delivered, and it having an effect on the target population. This delay means bioweapons don't give instant results (unlike nukes), so they're not very good battlefield weapons. This also means that if you suspect a biological attack you have time to take prophylactic measures, including immunisations against suspected agents or just increased hygiene etc. Or, of course, launch your own attack in response.
On the other hand, bioweapons are inherently difficult to control. Once released, their area of effect is out of your hands - they will spread and mutate over time, which is something of a problem if you or anyone else wants to be able to use that area ever again. It's also entirely possible that a biological attack will swing around and hit your own population.
So you simultaneously have a weapon that is difficult to control, imprecise in its effects, and gives your opponent time to retaliate once it is used; to say nothing of the possibility that it will affect ones own population as well. That isn't a very attractive combination for most purposes, and more or less makes them impractical for battlefield use. They might make good vengeance-weapons, in the same way that the threat of massive nuclear retaliation might have a deterrent effect, but deterrence depends on your opponent believing that you're crazy enough to actually do it (and is outside the scope of the question in any case).
I suppose that if there was a clear historical example of their use bringing a state to its knees (before or during WW2), that might give them the same sort of status as nukes do today. But really, from a practical standpoint nukes or chemical weapons are preferable for most purposes I think.