This is a question that the story does not address, so we have to assume that in Doyle's culture it would not have crossed a reader's mind. That leads us back to trying to understand how the society of the late Victorian period differs from our own.
Jabez Wilson, though he runs a pawn shop, a less than reputable business, is described by Watson as a "gentleman." He does own his own business, which puts him in the category of tradesman (generally not considered "gentlemen"), but he is clearly literate and has the portly size and demeanor of a Victorian gentleman, even if his status is a bit incongruous—he once did labor, Holmes deduces, so he must have brought himself up in the world.
While marginally so, he is on the "higher" side of the class divide. He has servants; he is not a servant. In those times, there was a notion called the "green baize door," which referred to the typical door that separated the servants' areas of the house from the masters'. The employers, though this was hardly written in stone, were supposed to stay on their side of the divide. A cellar—we are not talking here about a basement with a clubroom and electric lighting—but a grim, dark, unheated, dirt-floored, probably rat-infested place probably used primarily for storage, was definitely on the "servant's" side of the housing divided. This is indicated by the fact that Spaulding can "colonize" the place for his "photography" without it causing any friction. He is not stepping on his employer's toes by invading any space it is remotely possible he his owner would want to use. Spaulding is in the part of the house he can rest safely assured his employee will not visit.
To add to this, though his employer's life has been sedentary and quiet, Spaulding takes no chances. He wants to get Wilson out of the way so that he doesn't take it into his head to investigate the cellar from hearing noises—or just because Wilson has nothing better to do. That is why the four hours a day job copying the encyclopedia is devised—to keep Wilson busy, far away, and out of the cellar.