Because the music is a part of what *makes* a Halo game. Even with some changes (H2's inclusion of guitar, H3's emphasis on piano), core themes have stayed the same. Just like the Mjolnir armor changed from H1 to H2 - even if it was "different," it was still recognizable. If Halo Wars had the Mjolnir armor as a skin-tight stealth suit, it would instantly be recognizable as something out-of-place - the same goes for the music.