Hello,
There are two reasons. The small receivers do not have a recording capability. This is achieved through the telemetry backlink from the receiver. This equals a telemetry recording.
The transmitter card logs all information that is available to the transmitter including the data return from the receiver. At times it can happen that the backlink from the receiver is interrupted. Then there is no data available from the receiver.
The big receivers keep recording, so that after the flight you can see what happened and what might be the reason for the interruption. If the model crashes with one of the big receivers, you can afterwards tell what happened. Its like a black box in the airliners.
I hope this helps,
Gerhard