Curly

I have no idea, but for my better understanding it was due to stupid reasoning.

I think it was because Curly was so frail looking and weak . And they wanted people to remember Curly from his healthier days .

Two reasons have gained most favor, the most plausable, Curly really wasn't well enough to go back to work, his timing was off, he couldn't remember lines etc, the other is the Moe the Four Stooges wasn't an idea that would work.That doesn't fit with Moe's other actions he watched over the others like a mother especialy Curly, Moe was always worried about his finances and if there had been a way to keep him working he would have done it.
"Pardon me while I have a strange interlude"- Marx