Because quantum physics. A qubit isn’t 0 or 1, it’s both and everything in between. You get a result as a distribution, not as distinct values.
Qubits are represented as (for example) quantumly entangled electron spins. And due to the nature of quantum physics, they are not very stable, and you cannot measure a value without influencing it.
Granted, my knowledge of quantum computing is very hand-wavy.
But this is not just any abacus, it’s one that calculates all the results at once. That is a disruptive leap forward in computing power.