[mkl]

Two word surnames are unusual in English, and the "Le", "La" etc. could easily be misinterpreted as a middle name, leading to wrong names like "Yann Cun". English surname prefixes like "Mac", "Mc", "Fitz", "O" [1] are generally attached to the name that follows (sometimes with the second part capitalised, sometimes not), so there is an old and widespread convention being followed here. Nowadays it's easier to search for or use as a username too.

Yann LeCun spells his own name like that: http://yann.lecun.com/

[1] These are actually Celtic originally: https://en.wikipedia.org/wiki/Celtic_onomastics#Surnames