Why not 6 cards? I don't mind a bit for it. :yuk:
Because the full set of 6 cards was offered as a Kickstarter exclusive. The printer made too many of them, so Common Man Games is making them available. But because of the promise to the KS supporters, the complete sets will never be available for purchase.
There is a thread set up for folks who are ordering multiple sets to trade their duplicates and complete their set - but you can't buy it outright.