10-mans are way more line-up-dependent than 25-mans. It would be way more of a hassle to gather the groups together for a functional 10-man LFR than it is for a 25-man LFR.
You'd have to make LFR way more forgiving if you wanted it to be played by a random assortment of 10 players. One bad apple can hurt that bunch of people way more than your typical AFKer can hurt your 25-man LFR.