The incentives are different. Bioweapons are mainly destructive and not useful for achieving any aims that are sane (they don't secure energy, they don't increase local prosperity, they don't win land wars, they don't target political opponents, etc). Anyone who defects from banning them gains a target on their back and that is it. I can't think of a use for bioweapons.

Contrast to AI. Literally anyone who gets involved, from a teen upwards gains magical new powers across a range of arts and sciences as well as gaining new insights into any questions they have. There is a vague promise of utopia where it no longer makes sense for humans to work (probably won't work out that way, but whatever). Defectors gain massive advantages and can also maintain plausible deniability.