Coherency

To get useful results, a quantum computer needs all of its qbits to stay entangled with each other, until the entire group collapses into the result. With current technology, it is very difficult for a reasonable sized group of qbits to stay coherently entangled, so it can only solve problems that are also relatively easy to solve on classical computers.

If someone today were to figure out how to keep large numbers of bits entangled, then quantum computing would instantly be able to break any encryption that isn't quantum safe. It's not something that we are slowly working toward; it's a breakthrough that we can't predict when, or even if, it will happen.