Does anyone know what would cause a starter to not even turn on?

What is your question?

I have a 1990 Ford F150 with a straight 6 engine. The battery is in good condition and the wiring from the battery to the starter has been replaced, as well as the starter solenoid. Everything turns on when I put the key in, it just wont crank. The alternator is new and the spark plugs have been changed. I don't really know what it could be, but any advice would be great.