I read up on it yesterday after reading your question, and I completely forgot to answer you. It seems IBM is also working with the superconducting electrical circuit type qubits. They say that what makes their quantum processor work with so many qubits is simply many small improvements leading to big steps. Two things in particular stand out to me.

To do quantum computations, you need to interact with the quantum system. However, the easier it is for you to manipulate your qubit, the easier it is for anything else too. For these very small, high-speed, superconducting circuits, vibrations are especially deleterious to performance. IBM apparently delevoped a new alloy for their wiring that comes closest to being in contact with the circuits, for a suitable definition of "in contact". This alloy is optimized for passing current but not vibrations at 4 degrees Kelvin. This is the first improvement they highlight.

The other is a question of architecture. Usually, the circuits and the wires leading to them are on the same silicon wafer, in the same plane and on the same plate, if you like. However, IBM managed to split their circuit up in a way so that all the superconducting qubit parts are on a super cold, low-vibration plate of their own while the wiring to each qubit comes from above or below their silicon wafer instead of on it.

So they seem to be doing lots of stuff! Mind you, I did not read verification of their claims, but I think they sound reasonable enough, though the advances they make probably require a lot of other optimizations behind the curtains as well.