The banjo evolved to prevent the frame and skin being used as a bodhran:-)

My assumptions would be that for an instrument to be taken abroad by an emigrant it would need to be:

(1)played by the emigrant
(2)cheap, if it was expensive it may be sold to finance the trip
portable (if they had an expensive instrument would they need to emigrate in the first place)
(3)sturdy to survive the harsh conditions of the trip
(4)low tech - easy to maintain and fix or replace - the new land
would need suitable raw materials to hand
(5)fashionable - did the emigrant wished to be associated with an 'ethnic' instrument - links back to (1)

My guesses would be that there were few bagpipers in the general population that emigrated and that if bagpipes were taken there was not the skill to maintain the,; and the fiddle (which could be knocked up by a relatively unskilled person) was the fashionable instrument to play - a bit like the guitar in the second half of the 20th century.