I can’t speak for the design decision but although there are VCO’s that support wider stable octave ranges they won’t neccessarily have the same qualities - it’s unlikely to have had any real cost implication.
Sometimes a VCO will be chosen because it performs particularly well for bass, for example, and in that case the range will almost definitely be limited to the lower octaves.
It’s rare to get an opportunity to see this play out as well as with the Moog Minitaur + Sirin. Essentially the same synth but with a different VCO, they have very different strengths and sound.
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