I suspect a dual purpose for his not actually coming out and asking. He does sort of have a go at figuring it out, that's why he makes the list of things Holmes does and does not know - Holmes said that he only holds on to knowledge that's useful, so if Watson can discern the theme, he can decide what Holmes might do with that knowledge. I think too, though, that it just sort of suited ACD's purpose because he wanted the reader to wonder along with Watson, "What is this guy doing? What's with all these weird visitors for whom I have to vacate the sitting room?"