My read was Plenti attracted users that were not really profitable for the stores and Amex. There was also too much friction to use the service. I see both of these points are mentioned by others above.
I don't see it as incompetence, I see it as loyalty programs except for the airlines are a program that cost money and shared economies are hard. And those airlines who have milked the profits from their miles programs are going to be in trouble if they keep ruining the redemption value. I feel it is mostly worse for the consumer to have each company with their own loyalty program. A common currency with enough shrinkage options for the retailer and beneficial for the consumer might be a unicorn.
Rasheed