There was no obvious reason. There was only whine. In a 25 man guild you needed 2 tanks, 5-6 healers and rest was dps. This meant that some players got to experience real raiding without being absolutely perfect at their game. There was less pressure on indiviuals and a better sense of community. The 10 man lockout was for maxing out points on the weekend with some random pugs. People who wanted to min-max had that option and people who didn't want didn't feel forced to.
By the end of Wrath there were around 2 million raiders, more than now. The 10 and 25 merge meant that the number of Tanks and Healers needed to sustain raiders had to dramatically increase which wasn't possible. All of the people fortunate enough to be part of large and forgiving guilds were left out. The addition of LFR didn't solve this, it just added a toxic environment where no one cares about anything except himself. People don't organize themselves and don't know each other anymore.