Android 4.4 KitKat is designed to be able to run on older hardware and less powerful hardware too. Google dropped support for this phone because it is more than 18 months old. Developers however just couldn't accept this and decided to port the operating system themselves.
Developer Grarak released an early build of Android 4.4 just days after KitKat' source code was made public. The first builds of SlimKat for the Galaxy Nexus were released by the developer team behind SlimROms. This was all possible thanks to Samsung Galaxy Nexus' unlocked boot loader. This makes it possible for any developer to load his own software.
Android 4.4 KitKat is still somehow buggy but people that want to help with the development are more than welcome to install it.