why would a car not start after changing the battery, alternator, gas pump, and starter

Thanks.

Probably because you did not fix the problem. Is the gas pump running when you turn the key on? Describe what happens and what does not happen when you try to start it.

My daughter left the ignition on in my 2001 Ex. I was told by the locksmith that came out that as the battery slowly dies the computer can send out voltage spikes. 1 of these spikes wiped out my PATS and the truck would turn over but not fire a lick. I had to replace the reciever ring at the key switch and the PATS module under the dash on the firewall. The locksmith came out, reprogrammed the PATS, and she fired right up. The theft light was on fast flash and gave me a code that there was no communication with the chip receiver. Don't know what you're experiencing but that was my problem.