Why is the vote of no confidence on the prime minister for Conservative MPs only?

There will be a vote of no confidence in the prime minister of the UK today, but it is limited to Conservative MPs only and requires a majority of them to pass the motion. None of the media coverage I saw explained why the vote is limited to Conservative MPs. If the vote is open to the opposition as well, there will be a very good chance for the PM to lose. So what is the justification for this limitation? Why can't the opposition vote?