std::bitset is one of those “not quite really STL” types, like std::string and std::valarray, that tries to fit a couple of use-cases but none of them are “STL iterable container.” Arguably, the hint is in its name: whereas vector<bool> is clearly a vector, a sequence container, of boolean values, bitset is a set, a collection of small integer indices. bs.test(42) is the equivalent of s.find(42); bs.set(42) is the equivalent of s.insert(42); and so on. (As usual, these named methods throw std::out_of_range on error, and bitset provides operator[] for faster unchecked access.)
If you were going to iterate over the “contents” of such an object, what would you expect to see?
  std::bitset<1000> bs;
  bs.set(42);
  for (auto elt : bs) {
    std::cout << bs;
  }
Surely you wouldn’t expect to see “false, false, false, false, …”!