Accidentals.

Take an ordinary recorder. Play a tune on it in C. The twiddly decorations, the things that a folk musician who is not rigidly following dots do come from wiggling the fingers in a certain way. Ideally you want your twiddles to come from putting down or lifting up one finger at a time only.   Like the little bit everyone does on a D chord on the guitar, the top F sharp goes up to G, back down to F#, then to E than back to F#. Now do the same thing in A - still easy. Now E - not so easy because you have to get the little finger over to get the F# on the D string.

Now play your tune on the C recorder in (let's be brutal) F# or B major. All those little easy twiddles have become hard.

Same for a fiddle player - all the bits where they could double-stop with ease vanish when the key changes.