Why is ART not available on Nexus 7 (2012) or Nexus 10 running Android 4.4?

I just updated my Nexus 7 to Android 4.4 KitKat and was looking forward to testing out ART, but that option isn't available. I got word that the Nexus 10 also lack this option, but I can't find any info as to why that is. Anybody have anything to share? Of course, they would need an AOT compiler for that platform, but how big of a difference is there between Nexus 7 and Nexus 4? They are both ARMv7, although Nexus 4 is A5-based and Nexus 7 is A9-based (according to my research). Why would that make a difference?