There is no such thing as a free lunch.
Personally, I think the reasons are more than just Reach is not a multi-platform game. Games can overstay their welcome. Not counting Halo CE twice (with CEA releasing this year) Halo has had 5 games in 10 years, 6 if you count Halo Wars. IMO, that is more than enough for players to burn out on the franchise.
Add to it all the arguments that have been on these forums, and elsewhere, about Reach, and it's not really a suprise that Halo has fallen off the #1 spot. I am not saying Halo is a bad franchise at all, it's just an older franchise that has had a good run. Nothing can stay at the top forever.