Because of all currently "big" altcoins, it is the only one with credible anonymity.

DASH is only as anonymous as the tumblers don't collude and don't keep logs and that's highly doubtful with Evan in control.  ZCASH is not anonymous by default and has a very small anonymity set.

The smaller coins are too hard to get by for the victims.  Monero can be bought on several exchanges, it is traded a lot, and as it is essentially not traceable, if you withdraw monero from an exchange amongst all the traders, nobody will be able to point at you specifically.