This is my guess: sort of going off of what Big Chief said, I think that when Curly was in his prime, the writers and directors didn't really know what to do with Larry, so they mainly focused on the Curly-Moe dynamic. However, when Curly's health started failing, this forced them to give Larry more of a personality and more screen time (THREE LOAN WOLVES being the obvious example). By the time Shemp joined, I think that they were so used to the adjustment that they simply decided to stick with it, especially since Shemp was the oldest and at more risk of getting severely injured.