Last year they had a couple of special mini-events didn't they? I seem to recall Parallels coming out mid heist.
Anyhow - its special because you have to wait for it, its a holiday feel, this year especially. Every other month or something crazy would dilute the impact they have when they do hit.
Although that said me I'd love to see it maybe twice a year Spring and October, something like that. But am happy to be heisted whenever they're ready, not when my impatience demands.
Make me happy buy my books! (search Kercal on Play, Amazon and the rest).