There are good reasons, I think.
First, I have three .357 Magnums:
- An essentially-new-in-wooden-box S&W M27-2
- A Colt Border Patrol (1970's version)
- A S&W M60, stainless
I like .357 magnum; it's a versatile round.
But I like .38 Special, too: I believe it is equally if not more accurate (at least to 50 yards), is pleasant to shoot, and fun to develop loads for.
But I don't want to have to think about carbon build-up in a .357 cylinder while shooting .38's in it. If I'm shooting .38 Special, I want a dedicated revolver for it. Likewise, I bought a .44 Special GP100 even though I have a S&W M29 and a Super Blackhawk in .44 Magnum.
Besides, S&W has made some of the nicest and most beautiful .38 Special revolvers - the K38, Model 14, for example, which have a great reputation.