Could it be they wish to avoid the embarrassment of finding a passenger who should be excessed, but not have the means to do it?
Back in the day, travelling ticket collectors used to be armed with a low-technology pad, carbon paper and a biro, and as none of these required a battery, excess tickets could be provided. Do they carry similar items today for use in the event of the machine being inoperable?
Otherwise, imagine the scene:-
Guard - Excuse me, Sir, but this is a standard-class ticket and you are sitting in first-class. You owe the company £xxxx.
Passenger - Deary me, here's my debit card. Please excess me.
Guard - Err, umm, unfortunately my machine's not working. But will you promise me that when you leave the train you'll go straight to the booking-office to pay the £xxxx.
Passenger - Yes, of course, please rest assured I will.

