Why is std::bitset::size non-static

I can't possibly imagine why it was chose that std::bitset::size is non-static. It makes it much harder to get a constexpr size; you have to write something like this:

template<int val>
struct int_
{
   static const constexpr value = val;
};

template<size_t size>
auto getBitsetSizeIMPL(std::bitset<size>)
{
   return int_<size>{};
}

template<typename BitsetType>
constexpr size_t getBitsetSize()
{
    return decltype(getBitsetSizeIMPL(BitsetType{}))::value;
}

When if it were static all you would have to do would be

BitsetType::size()

and there would be no sacrifice of functionality.

Is there a historical reason that I am missing or is there a technical fact I am missing?