Fiddles are made to play in D - the notes are the same distance apart on each string.

Not true. However, fiddles sound especially good when played in G, D, or A because of the resonances with the open strings (G, D, A, and E). Most of the famous violin concerti are written in D. In general, fiddlers prefer the keys of D and G. Scottish and Shetland fiddlers also like the key of A because you can play double stops (drones) with the A and E strings, and the music sounds so good and LOUD. This is very useful when fiddlers want to be heard even though bagpipes are being played nearby. Another reason is that it's cheaper to pay one fiddler to play double stops than to pay two fiddlers.