Given the origin of the instrument I can guess why:
Mandolino/a (italy) → Mandoline (france) → Mandoline (UK original spelling) → Mandolin (English modern spelling)
Also, the kitchen mandoline is also from “french” (depending on your views about Alsace being french) origin. I have a similar model to the XIX alsacian ones, that I use to slice cabbages and lettuces.
ETA: I would add the idea is not new, and I’m pretty sure the japanese katsuobushi shaver is a lot older, but most people I know seem to point to the “mandoline a bois” as the precursor of the modern one 
