Why does initialization of vector of class with implicit string constructor from string literal fail?

Question

The following code fails to compile.

#include <iostream>
#include <map>
#include <vector>

using namespace std;

class mc
{
    string s;
    public:
    mc(const std::string s) : s{s}{};
    // mc(const char * s) : s{s}{}; // Adding this line would make it work in both cases
};


int main()
{
    vector<mc> a = {"Hello", "there"};          //FAILS
    vector<mc> b = {mc("Hello"), mc("there")};  //WORKS
}

Depending on compiler error is

no matching function for call to  mc::mc(const char&)
note: candidate: ‘mc::mc(std::string)’
note: no known conversion for argument 1 from ‘const char’ to ‘std::string’

or

/usr/include/c++/11/bits/stl_uninitialized.h:138:72: error: static assertion failed: result type must be constructible from value type of input range

Why does this error happen? There is an implicit constructor from string. If instead of vector<mc> they were vector<string> it would work.