So while it's been out for some time, a lot of enterprise code bases are only just now starting to move to Java 8, and developers are only now really getting their teeth into these sorts of features.
Also, Java 8 was a pretty huge release. Even experienced developers are no doubt discovering little new interesting features and tricks all the time.
It can take a while to move to new major language versions, especially when they have major new syntax changes; toolchains and IDEs take a long time to mature. Java 4 -> was huge, 5->6 was very minor, 6->7 medium (string switch statements, try-with-resources, and multi-catch), but 7->8 is another huge change. My guess is that Java 9 will be accepted much quicker than 8, since its changes are not going to be nearly as big as 8.
newer, trendier languages like Python, Ruby or C++? ;)
edit: now with parameters and custom literals