The fact that survivability of TIE pilots was really low, and low survivability = lack of experienced pilots.
Well yeah, so given that you have a flight element full of replacements with less than 10 hours on ties, low chances of surviving an encounter at the best of times and even if you get back if you engaged and took heavy losses and the convoy escaped Vader will probably choke you to death.
Hence, your best option is to defect to the Rebels or at least try to avoid combat, shift the blame on to someone else and try to get yourself posted to a quiet sector.
Edited by ErikB