The two cases are not parallel.
  a = sorted(b)
abbreviates
  a = list(b)
  a.sorted()
which occurred often enough to be a nuisance. With this proposal,
  a = reversed(b)
would abbreviate
  a = reversed(list(b))
which is probably less common and certainly less obnoxious than the two lines condensed by sorted.

A second problem: reversed already has a fallback default if (a presumably more efficient or effective) b.__reversed__ does not exit:

n = len(b)
a = [None]*n
for i,j in enumerate(range(n-1, -1, -1)): # reversed(range(n))
  a[i] = b[j]

(I believe this is more efficient, at least in C, than
  a = []
  for i in range(len(b)-1, -1, -1):
    a.append(b[i])
but it is hard to know.)

At what point, and under what conditions, would you introduce the second fallback?