What exactly is it that makes it a worthy replacement? (Yes, I'd like opinions on this; I haven't looked deeply at the guts of the matching algorithms.)

My guess: File size (in a brief check, Rainbow appears to be about half the size, unpacked), active development, and use of github (code-prettify is a few years old, hosted via SVN on Google code, and updated infrequently.)

However, prettify has a much broader range of languages already available for use.