Silicon etching process
You may want to check the chemistry stack, but as far as I can remember, it has to do with the number of atoms in the plane.  Using a 111, you have more atoms per plane, therefore the remaining channel has more atoms to remove in the same amount of time, which takes longer, but gives a stronger base for deposited oxides and metals to bond to on an atomic level.

Edit:  Here is a paper that may help you "see" the differences:
http://users.encs.concordia.ca/~mojtaba/Chapter1.pdf