The Russian Flu pandemic, one of the deadliest pandemics in recorded human history, raged between 1889 and 1895. London, with its huge population and quite frankly piss poor sanitation and hygiene standards, were particularly hard hit.
Knowing this, it lends another light than usual to why Watson was involved in so few of Holmes's cases between the autumn of '89 and Holmes's "death" in April '94. It wasn't that he didn't care about his friend or didn't want to get involved, but as a practicing doctor in a pandemic riddled metropolis he would have been overworked for a large part of the year while the virus resurged.
Also Holmes would have been hesitant about involving him against Moriarty, not just out of concern for his safety but because he knew his friend was already overworked.
It also offers another facet to why Watson was willing to sell his practice almost immediately upon Holmes's return and why Holmes asked him to.
Watson didn't have the best health to begin with, he did after all never fully recover from his case of typhoid fever. When Holmes returns in April '94 that winter's resurgence had only just begun to abate and this was just the latest one of a yearly recurring resurgence, resurgences that often took over half a year. This as well as Watson's twin bereavements (Holmes and Mary) could easily at this point have meant he had reached the end of his ability and capacity. Holmes might indeed be legitimately worried that Watson might work himself into an early grave if he doesn't take a prolonged break. A ironic role reversal that I don't think was entirely lost on Holmes.
So Holmes might have asked his cousin and fronted the money just to be sure that Watson did sell and quickly enough that he didn't get second thoughts.
All of this is brought to you by the theory @jeremys-come-to-bed-eyes showed me, that it was the Russian Flu that was the cause of Mary Watson's untimely death. Something that would lend an even more tragic twist to the whole thing.