Daniel Eran Dilger covers the challenges Android will face in moving to 64-bit technology:
(most) Android apps are not Linux processes; they are Dalvik executables that run on a Java-like virtual machine. Typical Android “.dex” apps are not native code in the way all iOS Cocoa Touch apps are... Redesigning Android's Dalvik/Java VM architecture to make effective use of a 64-bit processor is not a trivial undertaking.
There are also “Native” Android apps, often games, that run without using the Dalvik VM for performance purposes. This segmented native/virtual rift further complicates Google's efforts to make Android a truly 64-bit environment, even if it were interested in doing so.
Getting both native and VM apps to work in 64-bit while maintaining 32-bit compatibility for both types of existing software will involve memory management issues of its own, for a platform that already requires more RAM than iOS to work acceptably.
Delivering such a technically involved transition to 64-bit would also come as Google itself is turning its attention to Chrome, rather than doubling down on Andy Rubin's Android-centric strategy, which so far has primarily amassed significant legal problems related to its cavalier approach to intellectual property and built the company a fan base of users who don't like to pay for things, and in particular, software.
There's also another complication involved in Android moving to 64-bit apps. A key problem for Android as a platform is that developers are not actually exercising it. It's not a premier platform for novel, interesting apps. Additionally, developers aren't even taking serious advantage of existing Android 3.x/4.x features, in large part because the largest fraction of the Android installed base is still suck on Android 2.x.
How many years would it take for a new 64-bit edition of Android to accumulate even a tenth of the platform's userbase, given that the majority of phones sold by Samsung and other licensees are low end devices?
In any event, it appears that Apple will enjoy a year or two exclusive in selling a 64-bit iPhone and (assuredly) iPad, and be able to transition its library of App Store apps to 64-bit savvy long before any 64-bit alternatives reach even the bleeding edge of adoption on any other mobile platform.