The problem is they have to choose one game to cover for full highlights. No-one (ITV/Sky/BBC) in that situation will choose a League One side against a Premier League team because, 99 times out of a hundred, the PL club will walk it, which nobody wants to watch.
In the Carling Cup the TV companies tend to go with matches between the bigger clubs. The only people to buck this trend have been the BBC - who don't have to worry anywhere near so much about viewing figures - in the FA Cup. And even then they'd usually have had a glamour tie to go along with the potential upset, that more often than not is a massive switch-off 5 minutes in.