Why can't they track the iPhones (Smartphones) of the people on missing plane?

Many of them would have one and also activated locate my iPhone application. And they say your movement can be tracked on smart phones.

They were over the ocean at 35,000 feet. Not to many cell towers can pick up a signal under those conditions.

In order for the phone to be located it needs to be working and active.

The problem with using find my phone would be many. Firstly the phones are most likely destroyed in the crash and therefore not working anymore. They may be many miles under the see, and so can't receive a signal. And of course phones are switched of during a flight, so they are not active.

Maybe airplane mode makes it hard to trace

One, it was in the middle of "nowhere", as in nowhere in range of the IPhone tracking.
Two, they are in the ocean, if they weren't broken from impact of the plane hitting the water (and when a plane traveling that fast hits the water, it's like hitting a very hard and thick metal wall), then they would be "drowned".

Why can't they track the whole plane? The tracking satellites in orbit above should have detected the descent vector of the plane.

If the phone is restored the locater app will not work.