When they play they show nothing, in training they show nothing, they aren't as fit. Why would you pick them over the regulars?
It's curious how you accuse others of conjecture and then come out with stuff like this. As for them not being fit, he's not giving them gamer time to get them fit.
Constantly not giving players any game time and then stating they'll be needed during December is very poor management. If Irvine does suddenly start picking these players, and he's implied he'll do things before and then not done them, it'll be very difficult for them to go into games cold.
Although it is conjecture, one reason why Irvine doesn't select them could be that, if they did do well, he would be under pressure to keep them in the side and that would ruin his overall defensive "masterplan".