A serious scanner for 135 is affordable while a serious scanner for 120 is difficult to find and very expensive to buy.

You can walk with two or three 135 bodies without trailing a cart.

You can always have a compact 135 camera with you. There is no Minox, Rollei 35 or Yashica T3 in MF.

You can take 38 pictures without changing roll (the light or the occasion changes just while you are changing the roll, but that nuisance happens much more often with MF).

You can take pictures without appearing "professional" when you need not to draw attention.