Sherlock Holmes is immortal. The John Watsons come and go - the little details about their lives differ, but the general personality characteristics stay the same. John Watsons are literally the ONLY ones in the world capable of putting up with Sherlock Holmes. But they keep being killed (by Moriarty?), and Sherlock is desperate to stop the cycle. He LIKES this one, dammit - his current Doctor Watson is cleverer than most, has a delightfully dry sense of humor, and is actually tolerable for long stretches of time. Plus, it's such a bother to arrange a meeting with yet another John Watson . . . much better to figure out why all the previous Johns had such a high mortality rate and address that instead . . .