Simple answer-the acoustic guitar isn't very loud, compared to the fiddle, so in "traditional" music, it pounds out chords, while the louder instruments carry the melody and do most of the soloing.

It may be easy to keep up with the fiddlers when you play an acoustic along with a record, but it is a very different matter when you try to take the lead in a session, or in a band!

Plug the guitar in and it's a different story.