Decoherence
The quantum state leaking into the environment and turning classical — the single biggest reason quantum computers are hard to build.
A qubit must be isolated to keep its superposition, while still being controllable and readable. Every coupling to the outside world that makes this possible is also a path for information to seep out. Vibration, thermal radiation, magnetic fields and cosmic rays all contribute.
The time available is measured by two quantities: T1, how long a one stays a one, and T2, how long the phase survives. For superconducting qubits that is tens to hundreds of microseconds. All computation must fit inside that window, which is why quantum error correction is not a refinement but a prerequisite.