RNG. It’s available as one of the things that may pop up but it could very well not pop for months on end if that’s the way the numbers work. If you wanted a guarantee, you could have bought it when it was readily available. You didn’t, but now it’s just a waiting game and then the game of fighting everyone else who will swarm the BMAH once it does appear.
It’s on the loot tables. When it’ll appear, is anyone’s guess.
Also, here is some clarification that it is still available as a stand-alone item outside of the container.