Why can I charge me iphone with my car's usb port but not my ipad?

Because the iPad requires a lot more power to charge than your iPhone, and typically car and computer USB ports simply don't proivide that much.

Its the same reason your iPhone comes with a 5Watt charger (and even then the iPhone only uses part of that) while the iPads come with the 10 or 12 watt charger.

They just need more power.

The car charger has to output 10watts to charge your iPad if it outputs only 5watts then it will only charge your iphone it will not be able to charge your iPad

If your car has an AC outlet or you get a DC-to-AC converter, then you can probably use the iPad's charger. It's possible to use the iPhone charger or even probably your car's USB port, but the iPad will charge much more slowly.

Regards.

Who's car has ac power ?

There are many cars with AC outlets. 2013 Ford Explorer for instance.