Well, Halo fans, the wait is finally over. Halo 5 has finally been blessed with a release date of Fall 2015. In regards to why we have to wait until 2015, Frank O'Connor the Halo Franchise director at 343i had this to say:
"It's a really big project. It's a big project in terms of moving over to the Xbox One. It's also bigger in terms of scope and scale in the size of the already quite big Halo universe."

One of the parts of this project will be the brand new engine that game is being built on to help it utilize all of the power that the Xbox One has. This engine will also be the foundation used for other games in the series on the console. O'Connor elaborated about the new engine for Halo 5:
"With Halo 4 we talked about how we had gone back to the drawing board on a couple of features of that engine, but it was really an iteration of the existing Halo engine. This is a new platform with much more capacity and much more power, and on a different architecture."

What is interesting, is that 343i's General Manager ,Bonnie Ross, said this at E3 2013:
"...Today we're here to announce Halo's debut on Xbox One. A Halo FPS for a new generation of hardware that will take full advantage of the power and flexibility of Xbox One. A Halo engine enhanced by the power of cloud computing, dedicated servers, and for the first time on console – a Halo experience that runs at a blistering 60 frames per second. In 2014, Halo combat will truly evolve, and your journey with Halo on Xbox One begins."

Ross's quote implies that we will see something in 2014 that has to do with Halo, whether that be the highly anticipated and rumored Halo 2 Anniversary or something else unknown. If I'm speculating correctly, fans may finally get that Halo 2 Anniversary and be able to relive the memories.

My take on all of this, I am not surprised that Halo 5 is being released in 2015. If it was released in 2014, I feel that it would be rushed and would not be as good as it can be. However, it excites me that they finally have given us, the fans, a time frame for a release of it. As for the Halo 2 Anniversary speculation, I do believe that it is coming since she did directly state that the journey would begin on Xbox One in 2014 as opposed to 2015. This said, we will have to wait until, most likely, Microsoft's E3 Press Conference this year to get any facts about it.