These phones have lower specs with 512MB of memory available, and Gingerbread is what is required to fit within those tech requirements. That presented a technical challenge for Google in terms of how to build KitKat in a way that it can bring those older and lower-specced devices up-to-date, to help provide a consistent experience across the entire Android user base.
A new API in KitKat allows developers to determine what amount of memory a phone is working with, and serve a different version of the app to each, making it possible for the same application to run on the earliest Android devices. With KitKat, they have taken it and made it available on entry level phones. They have one version of the operating system that will run across all Android smartphones next year, as explained by Android chief Sundar Pichai said at a Google event today.
Google wants to get everyone on the same platform, and wishes to end the fragmentation problem. It will be up to manufacturers to decide whether or not devices get the KitKat upgrade.