This is because, if you haven't been following, this agreement between GTR & the DfT is different. GTR are not a standard TOC like Virgin or First etc. They are a Subcontractor and being told what to do by the DfT.
Probably the latter, but as they are only partially trained (I believe), maybe didn't what to be in charge with the heavier flow of traffic if something went wrong (as in an Accident).
And if number 1, with the amount of services cancelled across the network (not just Brighton to Lewes) they would've known how short they would've been well before the commute TO the stadium.
I've noticed that is all you ever seem to contribute to these threads lately, even though there has been no foul play by the RMT in this instance, just incompetent management.
Paul Barber doesn't even believe the 'Staff Sickness' excuse peddled out initially by Southern. The fact they have since failed to mention it should tell you something too.