From a marketing standpoint they picked the six House gangs as Thier core game. They will keep returning to the core game but with new toys. It's the same reason why there are a bazillion Space Marines. They will throw house gang specialists at us gamers in plastic because it will sell.
They may hype up a special gang for a bit, like Spyrers or squats, but will keep returning to the Core of the game. It may take a while to come round as they get distracted with the latest shiny things but they always return to the core of the game like all of their games.
The only lagging house gang is Delaque with a Ashwastes plastic and that will randomly come up after a rerun of the Gangs of Underhive bounty hunters in plastic or a collection of routine hangers on or a giant floating baby Psyker or some other weird thing. Then next month some bizarre Delaque mounts will be there for no reason whatsoever. (They have actually forced themselves to highlight the Delaque in thier ongoing narrative as the other five houses have spent their strength on ash wastes wars, pilgrimages and civil war leaving House Delaque as the only relatively untouched House)