Now that Google wants to take back control of Android, after leaving Android wonder off in so many directions over the past few years, with many of those directions not being an improvement to Android itself, but rather a distraction, Samsung thinks they need to focus increasingly more on Tizen, and try to push it into the market.
Now that Microsoft, the owner of WP8, has also acquired Nokia, which already had over 80 percent of the WP market, Samsung has no reason to stay in that market either, which also leads to them to focus more on Tizen, as an alternative to Android. However, Samsung will only try to push Tizen into the super-low-end to mid-range markets, for now.
The fact that they've added so much stuff to the Nature UX UI for high-end Android devices probably also means that they couldn't port the same ROMs to lower-end devices even if they wanted to and I don't think that Nature UX on KitKat would make that much of a difference, and by that I mean that I doubt KitKat+Nature UX would work very well on 512 MB of RAM devices, at this point. Stock KitKat? Sure. KitKat and Nature UX? I'd have to see it to believe it first, unless it's a very stripped down version of Nature UX that goes to high-end devices.
According to a slide Samsung showed off recently, Tizen (Full) can also run on devices with 512 MB of RAM, and since it's a new OS, they probably haven't had a lot of time to add too much stuff to it, so it may work pretty well on Samsung's 512 MB RAM devices.
Another interesting thing from Samsung's slide is something called "Tizen Lite". From the name itself we realize it's something quite different from the full Tizen itself. It's a more bare-bones, less polished, and going by the slides, pretty incomplete OS, too, that should work well "by itself" even on devices with 256 RAM (apps themselves, browser tabs, could still occupy as much RAM as ever, making the OS pretty limiting on 256 of RAM).
There were some discussions a few years ago about how bad it would be if Android split into 2 versions: a normal, highly polished version, and a "lite" one, that would focus on lower-end devices, and most people agreed then that it would be a mistake to have that, as it would fork Android into 2 different platforms. In a way, that's kind of what happened with Gingerbread and Android 4+, which is precisely why Google worked so hard to fix that, and create a common platform for both low-end devices and high-end ones, with KitKat.
Samsung alone can't make Tizen Lite, nor the full Tizen very successful, unless it co-opts other OEM's to adopt it, too, but with KitKat, Android 5.0 coming soon enough, and ART being enabled by default, I doubt too many OEMs would be tempted to go with the unproven Tizen instead.