Because your combination meter (speedo) has failed. Here's how to turn it off:

Place the car in Park
Insert the fob into the slot
press and hold the power button for several seconds until the car turns off. When you can remove the fob, you know the car is off.

The combination meter is the unit that displays your speed, fuel level, etc. It also controls ALL the dash lights except the check engine light. It is something of a computer as it passes data between the other computers.

The reason your car won't turn off is because your car doesn't know it's stopped. When the combination meter fails, the car no longer has any speed data, so it won't allow you to turn it off in the normal manner.

These failures are very common and were covered under an extended warranty program that has expired for most if not all years (2009 might be covered).

They rarely fail outright and are intermittent in nature. You may have it fail once, and then it works fine for months. That's what happened in our case. Our 2008 failed several times in one week. It then worked almost trouble free for 10 months. After 10 months, it almost never worked. We finally replaced it.

It should be replaced as soon as possible to avoid 1) potentially running out of gas because you can't see the fuel level and 2) it's like disconnecting your odometer - it no longer logs miles.

Lastly, if you are a battery customer, it could void your warranty. Running out of gas can destroy a hybrid battery due to running it empty, and that's regarded as "abuse or misuse" and violates the terms of the warranty.

The dealers charge $900. We charge $275, or you can DIY by ordering it from Texas Hybrid Batteries.