Remember the Google/Oracle lawsuits over the use of Java technology in Android? Bringing in JVM changes from newer Java releases risks giving Oracle grounds for another lawsuit (covering the patents and whatever copyrights may apply to the new features), and it's not likely Google wants to run that risk.

(This is why the Java 7 features that they have adopted are the "syntactic sugar" that doesn't require JVM changes, and leaves the code running in the phones as-is.)