My understanding is that the fiddle was the original instrument on which blues were played. Fiddles were far more common than guitars. Every household with any musical interest at all had a fiddle and guitars had not yet become common. The fiddle has the advantage of being fretless so it can play all the notes “in the cracks,” necessary for playing the African pentatonic scale and for making the slides one associates with the blues.
Note that much of what we know as white country fiddling today originated with African American fiddlers who played for the local dances. The African fiddlers had at least as much influence on country fiddle styles as did the Scottish and Irish traditions.