FOU: Field of Use Restrictions was the original sticking point.
The historical problem, with Apache, the Free Software Foundation, and Google were the Field of Use restrictions on Java-SE.
While Sun, and then Oracle, were busy 'open sourcing' Java, they still required all 'licensed implementations' to pass a test suite, and that test suite contained Field Of Use restrictions, that prohibited the use of Java-SE anywhere but the desktop. This was doubly true of mobile as Sun/Oracle _licensed_ Java-ME (as in definitely _NOT_FREE_) for real money. It was one of the reasons behind the founding of Harmony and the reason Apache left the Java Community Process when Sun/Oracle refused to provide a FOU free license.
OpenJDK is licensed under GPL + classpath exemption, Harmony was licensed under the Apache license. There's a lot of speculation as to whether Google itself was wary of the GPL, or it feared phone manufactures would be. Straight Java-ME kinda sucked for mobile (really). Straight Java-SE also had it's issues, but was used by many more developers than Java-ME would ever be. So, Google based the development environment under a slightly modified Java-SE (the Apache licenced Harmony project) that compiled to run a on a custom mobile optimized VM, Dalvik. Since everyone (well all of the honest/sane people) understood that API's weren't copyrightable, and the rest was Apache licensed they were good to go.
Unfortunately, Oracle is well Oracle, Intellectual Property [ugg I hate that term] litigation in the U.S. of A. has gone completely off the rails, and Oracle convinced I.B.M. (the most major of the Harmony backers) to jump ship and support OpenJDK effectively shutting the project down. This leaves Google NotJava stuck in Java 6 land.
As long as Google wants to keep development in Java land, isn't afraid of GPLing their android 'contributions' and can get the needed changes into OpenJDK, it makes sense to switch over to OpenJDK. This will allow them to progress to later versions of Java (Java 8 anyone?). Putting an end to Oracle's litigation has to help as well.
Now if we can only get the rest of the ecosystem to fully support OpenJDK then maybe it'll get somewhat closer to the "write once, run anywhere" ideal and away from the current "write once, debug everywhere, kinda sorta run somewhere" world we live in now.