Because the seat doesn't appear in the truck magically.
You have to engineer the seat, test overall quality, obtain enough components and materials for mass production, modify the interior to accept the seat, train the workers who will make the seats, train the line workers who will install the seat, physically modify the assembly line, integrate steps to the assembly line process, update maintenance manuals, inform repair centers, get safety approval and data...
... All of that takes time and money. If not enough people buy it you've just burned a pile of cash.
