There's no convention, in the usual British constitutional sense, which regulates party leadershhip. The monarch invites somebody to form a government who can command the confidence of the House of Commons, which is normally the leader of the party with a majority. But the leadership of the opposition is even less well defined, and isn't appointed by the monarch.

So, the short answer is that Attlee kept his leadership because he didn't resign it and the Labour party didn't force a contest. It has been common in recent years for leaders who lose an election to stand down, but this is not a constitutional requirement and it was historically common for a losing leader to stay in office and try again.