Let's say you lose a premade night of 3 games 3-0. Everyone would want some change, because nobody enjoys losing. But what would be better: to change comp everytime you lose in hopes of finally finding the right comp that suits your players, or to keep on practicing the comps that generally is viewed as the best, in hopes of getting better synergy and team-play.
This is somewhat off-topic, but not really. I used to play Guild Wars 1 before coming to WoW. For those of you that don't know, Guild Wars 1 was primarily a PvP game, that also had some PvE, as opposed to WoW which is the other way around.
One of the things that I really enjoyed about Guild Wars was the dynamic state of PvP. Due to its skill system, where a each character could pick 8 skills out of what seems like hundreds. Teams would make intricate builds where each players skill bar synergized with one another—almost like picking a deck for Magic or Hearthstone. Every once in a while, a build would get so good that it would become the prevalent meta build. Once that happened, the really good teams would make a build to specifically counter the prevalent meta build that everyone was playing, and tear up the ladder.
Arenanet also did a really good job balancing specifically for PvP. If a skill broke the game, it would be nerfed fast. But that really another topic.
I guess what I'm going on about, is that we have some elements of wow that don't allow the game to be balanced at any level, and that makes for stale comps that stick around for an entire patch. One such problem, is that of Rdruid as being the only viable FC due to its bear form damage mitigation, and the fact that being a non-tank spec, it doesn't take the increased damage that tank specs take in pvp (worst mechanic ever). This makes it so everyone is going to run Rdruid, automatically pigeonholing everyone into using one druid for that role. Also, take into the fact that balance is so ridiculously strong that it's basically mandatory, and add in the fact that ferals are so OP that they had to be banned and you've taken out a lot of options for comps.
I'm no professional strat-caller, but I've played a lot of games. It seems to me that it's the state of that game that forces people into obvious comps, and not so much the player base of 19s.