garyb wrote:Because that's how the name is pronounced?
+1. IMHO the question is somewhat ill-posed - it's unlikely that the spelling came first. Better questions would be 'Why does word X have this particular phonological form' (with an answer from etymology), and 'Why is word X spelt in this way (which doesn't correspond in the usual way to its phonological form)' (with an answer from the history of the language's orthography).
Having said that, I don't know the answers.
Maybe because it entered the language after final /s/ was lost in spoken French, and the word and its spelling were borrowed together?