This is great point, and I'll extend it by claiming that there's a more general physical principle underneath: that it's significantly easier to build bistable

This is great point, and I'll extend it by claiming that there's a more general physical principle underneath: that it's significantly easier to build bistable systems than tristable (or higher) systems, so much so that it makes up for the fact that you need more of them. This is far more general than electronic systems (e.g. quantum computers follow the same principle - it's far easier to build and control qubits than qutrits/qudits). (technically, it's even easier to build systems that have a single stable configuration, but you can't really store information in those, so they're not relevant)