Silent films were considered commercially worthless once sound came in (Chaplin and a few others being the exception), and nitrate prints disintegrated quickly (another reason why so few are left, and why many 1930s sound films are also lost).
Silent westerns in particular were forgotten because they had no commercial viability for being shown on TV once that came in. Also, according to William K. Everson's book "The Western", it was conventional wisdom in 1929 Hollywood that westerns would die off with sound. (The early talkie era would see fewer westerns made than at any time until the late '70s.)
It was the Depression and audiences' desire to escape its horrors that resurrected it.
The comedy stars associated with Hal Roach Studios, like Laurel & Hardy, the Our Gang (Little Rascals) series, and Harold Lloyd (although Lloyd was no longer associated with Roach by the time sound hit) managed to continue making films into the sound era, and sound proved to be a blessing for W.C. Fields - for the first time audiences could hear his wisecracks on film (not just on the vaudeville stage). Harold Lloyd's relative obscurity has to do with him controlling the rights to his feature films (both his later silents and his sound films, the only exception being his final film, the Howard Hughes-produced "Mad Wednesday" from 1946) and so they remained out of circulation for years. As for Buster Keaton, he lived long enough for his older work to finally be recognized while he was still around to enjoy the recognition. Keaton's career was not so much a casualty of sound but of studio politics - he stopped producing his own films and signed a contract with MGM which saw him as "Groucho Marx and Al Jolson in one man" and put him in some very cheesy early talkies which stiffed. Keaton's own marital difficulties and alcoholism didn't help. BTW, I have seen some of Keaton's talkie shorts from the '30s and '40s and those are unfairly neglected. Read a bio of his life or see the documentary Kevin Brownlow did about him - fascinating stuff.
Roscoe "Fatty" Arbuckle's real demon was mostly severe alcoholism. However, William Randolph Hearst exaggerated the extent of Fatty's alcoholism and "perversions" in our history, and in a timeline in which he isn't on Hearst's side, Hearst would smear him in a similar fashion if he runs for political office.
It's hard to do a what-if in which Fatty never goes to the St. Francis, as he probably wouldn't be remembered to the extent that he is if not for the scandal. He'd be just another silent comic/vaudevillian popular in his day who got forgotten with time like Harry Langdon or Joe Penner.
Admittedly, I have only seen ONE film with Arbuckle ("Coney Island", a comedy short in which Fatty was paired with Buster Keaton - probably because my interest in Keaton exceeds my interest in Arbuckle), but that's probably more than most people my age.