I don't think it's the mandolin's [frequency] range as much as the relative lack of sustain and compressed dynamic range, at least in most of what folks have at their disposal, that has limited its broader use/acceptance as a solo instrument.
The mandolin is, after all, pitched exactly the same as a violin, which has more than a few solo pieces. Some of those solos work on the mandolin (often with some adaptation, like use of tremolo), but many just do not translate as well, and don't generate the siren call of the devil's instrument.
Me, I'm liking not being one of 20 guitars at a jam, and having a space, even if shared with the fiddles, that is not contributing to the mud.
