Well for one by making a car a plug in - you add batteries making it further less economical once the juice is gone and you are running on gas
Then you are aadding to the upfron costs. .hybrids are already expensive, and adding more batteries, wiring and whatnot will further up the price.
Third, Ford is still struggling to figure out how to make the system such that something like an electric surge will not toast your 40k vehicle . - or simply damage something that would be claimed as warranty defect.
And finally, the 80mpg is very misleading.. I am waiting for people to finally start reporting the "miles per megawatt" of their electric engines so wee can see who much they use.. in regular hybrids this is not necessary, since the electricity is free, but with plug ins, you are actualyl PAYING for the electricity going to your car..
If you consider that running your Central air can rack up good $200 mothly electric bill, how much do you think charging a huge battery pack every day will run you....
Plug - ins are a good idea, but for now a lot of people have their reservations .. because of the fact that they are simply shifting the cost from GAS to ELECTRICITY .. plug ins are a super niche vehicles.
Of course, this is besides the fact that Gas electric hybrids are pure and simple fad... a simple Diesel engine could beat Toyota's precious invention ... I am waiting to see which one of big three will finally bring over a good 4 or 6 cyl Diesel to be dropped in a midsize sedan, and put Camry Hybrid's 39mpg to shame by simple old school technology. (now imagine that diesel paired with a hybrid, putting the Prius to shame)
Igor