Probably because the CHAdeMO plug costs so much money that they have not been able to get the price down to a reasonable level to create a decent market for it.
I did push Nissan on this topic when I was on the LEAF Advisory Board and they basically said they do not want to be in that business, but that they would support efforts by third parties to implement such products.
Personally, I've always wanted to have portable unit that would put out a couple of kW. I've often wondered if such a low power level could be handled by a bi-directional version of the on-board charger. Such a thing could be very affordable since all the electronics would already be in the car and all you would need would be a specialized cable. What I don't know is whether any bidirectional specification for the 1772 port has ever been considered.