Is there a reason why the groundbreaking crypto-cracking could not be a quantum computer that tries all possible passwords/keys simultaneously? (I'm no expert on this subject.)
Yes, because this is not how quantum computers work. That is, they don't perform all computation paths non deterministically and then give you the 'right' answer. The answer that you get is a composition of all paths; it is not known how to use this to solve NP-complete problems.
On the other hand, most modern encryption systems are based on the assumption that factoring is a difficult problem. Famously, quantum computers can factor numbers efficiently. So a large scale quantum computer could break current cryptosystems, but there are in fact cryptosystems (such as lattice cryptography) that are secure against quantum computers.
