Superconducting qubits are regarded as promising because superconductivity is a <macroscopic quantum phenomena> of <Bose-Einstein condensate>[Bose Einstein condensation], and so as a macroscopic phenomena, it is easier to control and observe.

This is mentioned e.g. in this relatively early: https://physicsworld.com/a/superconducting-quantum-bits/[]. While most quantum phenomena is observed at the atomic scale, <superconducting qubits> are micrometer scale, which is huge!
> Physicists are comfortable with the use of quantum mechanics to describe atomic and subatomic particles. However, in recent years we have discovered that micron-sized objects that have been produced using standard semiconductor-fabrication techniques – objects that are small on everyday scales but large compared with atoms – can also behave as quantum particles.


 Superconducting qubits are good because superconductivity is macroscopic