Two reasons:
- The movs is not supported if rd or rs is r13 (the stack ptr register).
  Newer versions of GNU as reject it, older versions were probably
  emulating it.
- The purpose of setting the "s" flag on some operations is to
  enable 16-bit encoding in Thumb2 mode.  However, for "mov"
  it is the non-s form that has a 16-bit encoding; the s form
  is never more compact.