Excellent question with probably no good answer - there were certainly a lot of other common names judging by Church records and so on. You could come up with a Biblical reason for John, perhaps, but then you would have to explain why Matthew and Luke don't appear...

So to suggest some new songs:

Oh No, Brit, No, Brit, No Brit, No!

When Tony goes marching home ...

All alone with my Iain on the shore ...

Georgie, Porgie, Pudding and Pie .... oh, we've got that one