Why will a car not turn over if there is power and the starter and alternator are new?

Internal combustion engines need 3 things to start and run: air, fuel, and spark. First look over all of your fuses, to make sure none of them such as the fuel pump fuse, or the fusable link have burned out. Make sure your battery is not faulty, even a brand new battery could potentially be no good. Check the voltage with your volt or multi meter, it should read around 12.5v. Inspect the cables to and from the battery and alternator, but sure they are properly installed. Check your air induction system, make sure the air filter is nice and clean, no blockages. If your vehicle is equipped with a MAF (Mass Air Flow Sensor) make sure it is plugged in correctly. Likewise if your vehicle is equipped with a MAP (manifold absolute pressure sensor) be sure that is plugged in correctly. Visually inspect the entire length of the intake piping, for loose clamps, cracked piping, or gaps. Once you have completed that if your vehicle has a distributor system, remove the distributor cap and replace it if necessary. If you see any visible damage to the spark plug wires, or the age of them is unknown replace them. Next check your spak plugs for the correct gap as recommended by the manufacturer, be sure they are in good shape before reinstalling them. If the vehicle still will not start, try replacing your fuel filter. This is usually inline, though in some cases it is in the fuel tank itself. Once that is completed turn the key to the accessory position, in most vehicles you should hear the fuel pump turn on and prime the lines for starting. If you do, you should be in good shape at this point.