I think it's pretty silly how the CMs are responding to players complaining being "forced" to do Raid Finder by saying in no way is this content compulsory, it's just optimal.
Come on guys, we know you're smarter than that. For many players, being sub-optimal is not an acceptable option. We know you know this because we've seen whole game systems designed around this fact so many times in the past. All you're doing by feigning ignorance is insulting the players who pose these legitimate questions.
But to the people who ask, the reason is not hard to figure out if you think about it. Blizzard just can't admit to it.
Raiders are "forced" to do Raid Finder because bad players need to be carried. Raid Finder is designed around this idea. It's about preserving the illusion of competence and therefore self-esteem of the bad players; to keep them feeling good about the game and keep them playing. Pretty much every developer decision that unfairly inconveniences raiders comes down to that same reasoning: to protect bad players from realising that they are bad.