What the sales figures of Reach compared to Halo 3 basically say is that the Halo user base (or more specifically, the buyer base) hasn't really expanded much since Halo 3.
This was pretty predictable for a major, pre-established legacy franchise (dating back to a previous generation) known to sell hardware platforms. They drive early adoption sales from the main fans of these franchises. It's rare for a fan of any of these franchises (GTA, FF, etc.) to completely skip the first showing on a given console (and taking a pass on buying the hardware) only to buy the second showing and the console years later.
Looking at attachment rates for these franchises, they consistently drop significantly as the user base for a given hardware platform expands to include a lot more than the typical early adopter which is front loaded with core gamers. And that's even if sales increase over the previous chapter of a franchise.
If you were a fan of the Halo series, it would pretty much defy logic to skip Halo 3 and the 360, only to buy Halo Reach and a 360 now unless you were off planet, in a coma or in a bomb shelter/cave for the last three years.
