They really couldn't. If I remember correctly, US law requires that a dealer provide replacement parts for publicly sold vehicles for 10 years. If you have a small run of cars with very custom parts like the EV-1, the cost of maintaining that capacity is likely prohibitive, so you kill them.
GM would only lease them, not let you purchase. When they repossessed them at the end of the lease, they would not let you purchase them; they then proceeded to crush any that were not sent to museums.
My question is, what if GM had offered them for sale for $250,000 (I'm making up an obscene number) because they estimated that their service obligations would cost that much? How well would that have gone over with the enthusiasts?