There were attempts to make decimal computers using voltage from 1~10v for example.

The problem happens on fluctuations, electricity being lost through wires, problems with energy escaping... And this can cause big problems of a 10 of the CPU becoming an 8 until reaches the GPU. If your part receives a 9.3v, it's a 9 that gained voltage along the current, or it's a 10 that lost it? Or an 8 that gained much more?

Binary is easy because "if there are ANY energy here, it's an 1. Otherwise, 0."

Far more reliable, electromagnetic interferes very little, error detection and correction are far more easy.