Because it wasn’t very good and just wreaked of lack of ideas, It revealed nothing, recycled a myriad of voicelines from the game and kind of didn’t make sense (Busan has no defenses? I mean WTF)
People waited a long time for a cool new short and they got next to nothing. Overwatch is just ripe for a ton of lore and Blizzard seems unable to deliver the goods in any meaningful way.