If they release a Halo 2 anniversary come 2014, then don't be surprised to see a demand for a Halo 3 anniversary when 2017 rolls around and so on.
Also this isn't the interview I was looking for, but it may have to do.
http://www.wired.com/geekdad/2011/11/343-industries-part-1/#more-94546
I think we are very conscious that people have very, very fond memories of when they first played Halo, and that’s not something that we wanted to mess with.
Just to compound what Dan said, the magic of Halo and the “secret sauce” is the game-play, and if you moved it to a different engine, if you moved it to a different technology you’d lose that essence.
So essentially they didn't want to mess with any of the memories fans had had, but they decided to make everything exactly the same, and that is why it worked.