It's just (bad) luck. I've done, probably, 15 or so and haven't gotten it, while others have reported getting it on their first.
As a PSA, though, you don't need to be using the weapon for its Catalyst to drop.
Edit: I just counted out of curiosity, and I've actually done 28, lol.
English
-
That must feel bad, I just wish I wouldn't have that bad of a luck.
-
Yeah, it can get pretty disheartening, especially after struggling through some of those Mercury adventures. But I'm used to this kind of terrible luck. I don't know if you played Destiny 1, but it took me five or six months to get the Vex Mythoclast.