why is only direct initialization prohibited for class member, not for local variable?

Question

with respect to c++11 syntax, I understood that direct initialization of class member is not allowed(most vexing parse). I referred to this posting :

Why class data members can't be initialized by direct initialization syntax?

but I did not catch any clues about why local variable is not the case. for example,

int main(){
   int a(0); //no compile error
   return 0;
}

for above code, compiler doesn't report error and compile this successfully.

Any comments will be very helpful.