Missiles and chemical weapons are physical objects which one can reasonably presume to maintain at least some degree of control over.
Encryption algorithms are math, theory, information. They can be reproduced, copied, and transmitted at-will, for no cost.
If the difference is still non-obvious, google around for instructions to make ricin. Easy to find, and it's not illegal to share that information. Why should it be illegal to share any other data? And how could one reasonably expect to enforce it?
It's definitely illegal to commercially export information in those areas as well, at least past some level of detail. For example it's illegal to sell missile or reactor blueprints without approval. It's not just selling the physical objects (missiles or reactors) that's illegal, but also selling the information on how to construct them.
Encryption algorithms are math, theory, information. They can be reproduced, copied, and transmitted at-will, for no cost.
If the difference is still non-obvious, google around for instructions to make ricin. Easy to find, and it's not illegal to share that information. Why should it be illegal to share any other data? And how could one reasonably expect to enforce it?