Seriously, though, changing the key does sometimes change the "feel/character" of a song. I can't imagine something like Dona Dona done in D major, even though it is possible. Of course, minors should always be left as minors, etc. but some folks don't.

I also can't imagine RagTime Annie in anything but A--usually the "commonly accepted" key gives sympathetic resonances and easy drones which would be lost if the the key were changed. Sometimes it really doesn't matter--John Henry sounds fine in either D or G. Would be strange in F, though.

I think you will find that USULALLY a tune/song in a particular key is designed for a particular instrument. For example:
St. Patrick's Day in the Morn(ing) is a great tune, but it is a bugger for fiddle fingering in G. Plays easier on the fiddle in A.

Glass O'Beer in D has great whistle fingerings, but bad for fiddlers. Fiddlers prefer it in (something else)

It has to do with fingering patterns for each specific instrument. Key is not generally the problem, fingering is.