They charge a lot of money for real-time telemetry from a plane. This charge can frequently represent the entire profit for a flight for smaller carriers, so they don't elect to purchase it.
Just 10 bytes or so every 5-10 minutes.
And over land you can use cell phone tower, so smaller carriers (who don't go much over water) would not be impacted badly.
Actually over land you probably don't need anything at all, so this really would just impact those best able to handle the expense.
You don't even need Iridium, a shortwave radio transmitter will suffice.
I find conflicting information about the Air Asia flight, a Bloomberg article claims the plane wasn't equipped, other places claim it was, but the airline wasn't subscribing to the data it generated.
It's not clear at all whether this is related to the cost of the system itself however.