With the exception of spore forming organisms, most biological agents are quite fragile. Too much heat, too much cold, too much dry, too much UV alone or in combination can do in biological agents. Viral agents such as Ebola, Marburg, Smallpox do not survive outside of a host for long. Most pathogenic bacteria (such as plague {Yersinia Pestis}) do not lurk in the environment but are transmitted via a reservoir of hosts (typically rodent-rodent, rodent-human, or human-human via the infected flea). What this means is that unless there are people who are directly under the dispersal cloud of the ICBM warhead not much happens. Agents lying around on the ground, with the exception noted, will degrade rather rapidly. On top of this, if you fire off these missiles right away, you may end up dropping biologicals on an area which was/will be nuked or ends up in the fallout cloud which will inactivate a certain proportion of those agents that survive to hit the ground.
Weaponizing pathologic agents does not necessarily mean they are "new" agents, it means packaging them in a way that allows them to be dispersed by spray, artillery, etc. Changing the agents so they are resistant to antibiotics or unaffected by vaccines can be done to some extent. However if you change a virus (like Smallpox) so that the outer coat is not recognized you may change its ability to cause disease. Making pathogens highly resistant to known antibiotics is very risky unless you have some secret antibiotics that work, likewise vaccinations for "new" agents. Given the state of the Soviet pharmaceutical industry this is not a given.
It is important to note that unlike nuclear warheads, warheads with biological agents can't be stuck on a missile (or in a bomb or artillery round) and let to sit with exception of a few agents. Most need to be loaded fresh or they become ineffective and this means transport from where they are made to the missile site, depot, etc shortly before use.
All of this does not mean biologicals can't be used but their strategic use delivered by strategic systems is actually pretty marginal.