do a search for the terms "art versus dalvik " and you will get a lot of hits from good sources.
A word from Google
ART is a new Android runtime being introduced experimentally in the 4.4 release KitKat. This is a preview of work in progress in KitKat. It is available for the purpose of obtaining early developer and partner feedback. - Android developers
In other words, ART is still an ongoing project, considered too unstable to be introduced as a standard runtime. Luckily, there is no special hacking involved to get it running.
You can choose to use it by going to "Settings> Developer Options> Select Runtime" and choosing ART.
And this from a YouTube article:
Published on Nov 13, 2013
ART is a big deal. Standing for Android Runtime, it has the potential to make lower end devices blaze through apps, and will result in a smoother, faster experience for all Android users. Check it out! Only in Android 4.4.