Java 6 source was introduced without explicit intention and once detected was not addressed. Many companies would not use something whose vital dependencies change on a whim or oversight. A lot of shops are still using Java 5 compilers and have no immediate plan to upgrade. Some cannot upgrade conveniently.
Your comment might be misunderstood. While Guava now requires Java 6 to compile, it is not required to run it. Guava is still targeting Java 5. You can use it by declaring a Maven dependency or by adding the latest Guava jar to your project libraries... But yeah, it forces you to upgrade your compiler.
Kevin Bourrillion (Guava lead developer) just made an insightful reply on this subject in the issue tracker:
> Saw this bug referenced as "a reason not to use Guava."
> Does everyone understand that Guava is still perfectly
> *usable* on JDK 5, it just can't be *built* using JDK 5,
> and that part of the reason for that is *compiler bugs*
> in JDK 5 whose fixes in 6 were never backported?
Java 6 source was introduced without explicit intention and once detected was not addressed. Many companies would not use something whose vital dependencies change on a whim or oversight. A lot of shops are still using Java 5 compilers and have no immediate plan to upgrade. Some cannot upgrade conveniently.