Ubuntu developers don't enjoy maintaining the open-source OpenJDK Java implementation. According to James Page, "the foundations team have been reviewing whether we should continue to provide OpenJDK as the default Java implementation in Ubuntu; this package is quite a resource drain on both the foundations and security teams from a SRU perspective so demotion of openjdk7 to universe is being considered."
Ubuntu developers would use GCJ -- the GNU Compiler Collection's Java language support -- in place of OpenJDK 7 for building Java packages. The GCC Java support has been losing ground but now Ubuntu developers are looking to leverage it to ease their support burden. Most Java applications can compile with GCJ, but a notable exception to that is Tomcat.
Apache Tomcat is the well known Java Servlet and JavaServer Pages server implementation that by default doesn't build cleanly "out of the box" with GCJ. Thus the Tomcat server would also be demoted out of Ubuntu's main archive.
The discussion about whether to demote OpenJDK support within Ubuntu Linux and in turn also boot Tomcat out of main is still ongoing and the discussion can be found on the the Ubuntu-devel list.