Because one instrument may be piano - it can produce more notes.
And so might people do in this compo, obviously. One backing note/chord + one melody note sounds like several notes to me. Hence the confusion I think.
One voice is more challenging, is it not? (btw, I want to hold next compo as One Instrument, so you could write a solo instrument music, like for piano)
Yes, but it would be even more challenging if one could apply one note at a time - what so ever; meaning no backing music. However, the next compo idea sounds very interesting. One instrument compo - and you're free to do what ever as long as you stick to that instrument, correct?
The idea of OVC came from listening to a lot of traditional music. It's usually a song, so it's one voice, since a man's voice can't produce chords. And usually a song is accompanied by chords. There's the idea. I was adviced, next time I run OVC, not to include that chord rule, since the melody can be harmonised and chored in various different ways, so it's not essential, only gets things complicated.

I see. I've gotten a certain interest for classical music lately myself. I have listened mostly to rock, blues and some funk - even jazz. Not much classical, however, but I'm about to change it. I'm interested in music theory, and I'd like to explore the approach of the classical masterminds.