From my understanding the big advantages to S35VN is how easy it is to grind/sharpen/polish, and it's supposed to be easy to heat treat consistently. Those are all win's for the manufacturer when it comes to profit margin, quality control, and increased production. It seems to me like S35VN is a good choice if they are looking to keep profits high and get larger numbers to market and maybe even at a faster pace (A guy can dream!).
Elmax should be a superior steel both in toughness and edge holding, but in normal use I usually can't tell them apart if I'm not paying attention. To me Elmax is a bit more work to sharpen but it will take a finer edge, it stays "scary" sharp longer than S35VN, but S35VN holds a working edge longer and is easier to touch up. That however is just my experience from a pretty limited sample size of each.
When you consider the 0452CF is made from "lesser" steel than Elmax and it's still only $25 less MSRP than the 0562CF/0620CF with M390 that's kind of a downer. I'd happily pay $25 more for M390, but I don't consider S35VN steel to be a downside either, especially for a working knife.
It simply might be ZT listening to it's customers, lots of posts about Elmax and it's heat treat, lots have complained about wanting more knives and getting them to market faster. S35VN could be in response to that.