It comes from mountainbiking and I think the main reason is to allow for huge tires to pass through them when opened.
They also allow for more clearance between brake housing and wheel so to eat less dirt.
They aren't real easy to set up like cantilevers but once you get the hang of 'em it's actually not that hard.
Roadracebikes still have canti's but most hybrids use mountainbike gearing and brakes.