A steam loco was in some respects a living entity. As an engineer or fireman you had to listen to it to know how it was working, what it was telling you, what it wanted, and so on. Modern diesels are so computer controlled the engineer is in many respects a button pusher (which if railroads could figure out how would probably be eliminated).
Steamers had the romance of the rails behind them, nostalgia, memories, history, lots of weirdly moving parts, and so on. Diesels are smudge pots (ok guys - obviously I'm more then a bit biased, lol).
Ben
P.S. To all you diesel lovers out there - go ahead and jump all over me - I don't care (sez me with a big grin, lol).