The last stable version is Loki 0.4.1. Why the leading 0?
I think this question has been asked a lot on different forums. The primary reason they have stuck with the leading major version number as zero is "because of things [they] felt were not yet completed". We have an app store with a few "paid apps" only recently. Some hardware aren't still supported (passed down from mainstream Ubuntu not supporting them yet).
To be fair, Elementary has been stable over the years. While its developers isn't still confident about it being perfectly complete, it has brought a lot to bring Desktop Linux to users with little or zero knowledge about GNU+Linux.
I believe it's because it's still considered beta. the real question is why after being on the market for 5+years it's still a beta.... but I guess that's something that @DanielFore can answer