Symmetric encryption is quantum-resistant. Most modern asymmetric encryption (public key encryption) is not resistant to attacks with quantum computers because

Symmetric encryption is quantum-resistant. Most modern asymmetric encryption (public key encryption) is not resistant to attacks with quantum computers because modern public keys are based on something called "hidden subgroup problems" which can be solved easily with quantum computers (if they existed). However, quantum-resistant asymmetric encryption does exist, it is just not yet common or battle-tested. Of course, all of this depends (as usual) on complexity theory conjectures. Very plausible conjectures, but not yet strictly proven theorems.