People tend to forget that in the end, railroad employees were people. People will always seek the easier way to do the job, because why wouldn't they do so?
Now, if you were someone with a job devoted to steam only, then you'd wanna keep steam around. But an engineer or fireman had a much easier time with diesels.
I once had a neighbor who was with the SP at the end of steam and he said though he liked running steam, he loved running diesels because he described it as from going from using horse at work, to a pickup truck.