Probably because you have to play a ridiculous amount of games in the Uefa cup and all around eastern europe, and the prize money isn't worth it, compared with getting in the champions league. Someone like Spurs would have been far better getting in the champions league rather than going far in the Uefa, although they didnt end up getting into the champs league they were close.
But yes, I don't understand why more clubs dont take the League cup seriously. I guess it's because they want to give their fringe players games and it is also an opportunity to blood younger players.