I replaced the key cylinder and the ignition switch, why wont the car start?

It may be a security related issue. How many miles can you travel before the vehicle shuts itself down? If the vehicle shuts down before you can make any progress it is likely a security related issue. If a few miles can be driven before shutdown it is likely something else. Did you do the work yourself or at a repair facility? If performed by a facility, it should be under warranty.