Change in Sales Game of the Day (9/28) - Halo 3

[Reasonable]

Well... I'd say the following were the factors:

1 - 360 benefiting from Xbox (particularly in US and UK) starting trend towards playing PC centric FPS and MP games on a console, and therefore becoming the console of choice for online FPS

2 - Halo franchise popularity is on an upwards curve due to this increasing penetration of 360 into what was formerly a PC focused player base, and Halo is the flagship title associated with that

3 - Increase brand awareness and identification of Halo 3 as the obligatory FPS and online MP title for 360 - as a result, a large percentage of 360 purchases will get the game where other titles have dropped from focus to be replaced by newer titles

4 - Shift in the title itself from SP to MP, with Halo 3 really representing an almost exclusively MP orientated title with a short SP campaign tacked on, making Halo 3 a huge ongoing proposition on the 360 with the title almost always the number 1 Live title being played.

5 - It's good (not that good compared to other FPS/MP titles out there, but that's what's interesting) and more importantly it has the marketing and image to compliment it's strong gameplay

EDIT 6 - Huge marketing push, sustained via books, figurines, etc. really pushing the Halo 'Universe' (thanks to the others who reminded me I forgot to add this important point).

 

So I voted the growth of the franchise as the closet poll choice, although as you can see I believe it was a combination of factors.