Likely future proofing for the "internet of things". Eventually, home users may want to have different subsets of "things" on different subnets. (Media centers, light switches, kitchen appliances, security devices, mobile devices, inventory (which could be broken down into other subsets, clothes, food, toys), etc)
256 subnets may be too few for that.
However, from an ISP perspective, 65K /48's in a /32 means I'd need a lot of /32's to cover my user base. Yes, there are a lot of /32's in the first /3, but it's a lot of administration that could be removed if we just used /56's (which would give us enough subnets in 1 /32 to more than cover our particular user base). And the "internet of things" is a long way off from a budgetary perspective.