"At that time [1955], transistors were not yet available, but it was clear that the machine should not use vacuum tubes. Tubes have a short lifespan, and tube-based machines were idle most of the time because they were always being repaired. A tube machine worked at best for several hours, then it was necessary to look for another malfunction. Yuli Izrailevich Gutenmakher built the LEM-1 machine on ferrite-diode elements. The thought occurred to me that since there are no transistors, then you can try to make a computer on these elements. Sobolev, whom everyone respected very much, arranged for me to go on an internship with Gutenmacher. I studied everything in detail. Since I am a radio engineer by education, I immediately saw that not everything should be done the way they did it. The first thing I noticed is that they use a pair of cores for each bit, one working and one compensating. And an idea came to my mind: what if we make the compensation core do work, as well? Then each cell becomes three-state. Consequently, the number of cores in Setun was seven times less than in LEM-1."
(https://notesofprogrammer.blogspot.com/2010/03/blog-post.htm...)
The basic reason for binary is because it's generally faster, especially as you scale to smaller transistors with more noise.