The shell is easy to make in quantity. But bullets have a much larger investment and Winchester was (and is) only making .358 bullets designed to impact at slower velocities. A new magnum with bullets that are not up to snuff is a death sentence for that cartridge. A "358 Magnum" should have bullets of 250 grains or even a bit heavier, and the jackets should have shanks of about .080" to .100 thick with the ogives tapered down to a thickness at the nose of about .015" Design is not hard, but tooling is costly.
So when Winchester looks at the market, a 358 Mag is going to be marketed to the same crowd as the 375H&H, and they already made a 375H&H and everything that goes with it from brass to bullets to rifles.
So my guess is they looked at the potential profits and compared them to the cost of tooling up and found there was not enough potential for such a new magnum.
Today if the .358 bore is your bag, a wildcat would be easy and Swift, Barnes Nosler Norma and Woodleigh all make perfect bullets for it. So it would be a fine cartridge I am sure.
But as was pointed out by others, it would be no finer then the 375.
