When Halo 3 launched in 2007, There weren't really big games similar to it on console. If you want FPS multiplayer, go and buy Halo 3. It wasn't until that year we saw COD4 and then the game followed and the genre became the most successful. After Halo 3 the franchise started to decline slowly.
Another reason that affected Halo franchise is bundling.
Xbox One situation and the mistake Microsoft and 343 did with Halo 4 and MCC is the main reason for Halo 5 decline. I don't think milking was the reason.
2007: Halo 3
2010: Halo Reach
2012: Halo 4
2015: Halo 5
Bundling and digital are other reasons for the decline in numbers. Halo 3 was the only Halo on 360 & One era to get a big push from bundling.
In short, the reasons for Halo decline, in order:
1- Xbox One and the way they handled multiplayer in previous games.
2- Competition.
3- Digital and bundling.
