Because then everyone would have their full set within a month and be bored even sooner.
I've done LFR I think three times (twice for the first four bosses, once for the last one), and I'm already bored with it. Not because I don't like the content or anything, but because LFRs are worse than random BG's for trolling and raging and BS in general. Case in point, just tonight, during my first-four LFR, there was a nerd fight during Morchok. Then during the ooze boss, somebody pulled him while we were doing trash and immediately dropped group. I then lost an Agi ring to a Boomkin who spent the majority of the Hagara fight raging against two priests who Life Gripped him into a bad spot and let him die. Then for a Fall of Deathwing LFR just a little bit ago, people who couldn't even roll on the loot that dropped (3 2h swords and a Vishanka) were raging about people other than themselves getting loot and how they're so much better than everybody else. I guess I shouldn't be surprised, knowing John Gabriel's Greater Internet F@#%wad theory.