Because they will simply demand more and more? This is the first time they've had content denied to them because they actually have to make an effort. Compare the content specifically for casuals and compare then content specifically for hardcore players, the list aren't even close to even. You had 90% of the content at your fingertips yet would not even make an effort to do the last 10% and instead spent your time whining on the general forums.
Furthermore having all content queue-able will lead to people being much less social. I've touched on this before, but the so called toxicity that casuals complain about in mythics come from their faults of wanting all content queue-able and cross-realm raids. No-one has a server reputation anymore since they can do 90% of this content by joining a queue with random people(Heck it's 99% now since CoS/Arcway/Kara are now heroic)
I've played this game a long time and it's obvious when the community started to fall apart and that was the introduction of LFD.