As I understand it, Doyle was tired of Sherlock Holmes at this point in his career. That, I think, is the most common explanation for the rather sudden demise of the character in the final story.
When I started reading it, I didn’t know that this volume contained "The Final Problem"; but I could definitely tell that Doyle’s heart just wasn’t in it. While the writing is on the same level as the earlier works in the Holmes canon, the mysteries themselves are rather staid and pedestrian. You very rarely see modern adaptations of "The Greek Interpreter" or "The Adventure of the Yellow Face" (though I applaud this one for its Victorian celebration of racial equality; and the line "I am not a very good man, Effie, but I think that I am a better one than you have given me credit for being." was downright heartwarming).
It gets so bad that my spouse couldn’t tell the difference between "The Adventure of the Red-Headed League" and "The Adventure of the Stockbroker’s Clerk" when I was describing the latter: they’re roughly the same stories with the final twist swapped out.
The most egregious problem with this collection, though, is "The Final Problem". Here, Doyle was intending the great detective to make his curtain call: but, the entire story is just about Sherlock failing to run away. In dialog, he says that he’s done the finest work of his career over the past several months, but we don’t know what steps he took or what deductions he made. I don’t know if the aphorism "Show. Don’t tell." was around in 1894; but if not, it would’ve behooved Doyle’s editor (did they even have what we consider modern editors back then?) to coin it. There was absolutely no substance to this story. I, for one, am grateful that it turned out to not be Sherlock’s final problem: not because I want more of the detective (though I do), but because I think he deserves a better ending.
There are some gems, though. "The Naval Treaty" was fun, and "Silver Blaze" had a delightful solution. And ultimately, even the worst Doyle-penned Holmes story is pretty good. It just falls down in comparison to its siblings in the canon. I think we can all agree that that is pretty tough competition.