To make you sub more than 1 month if you want to do it using LFR.
Also, in the past, some people would feel forced to do LFR to get tier pieces to complement their tier stuff from normal+ (keyword: feel), so by delaying the LFR, those people would complain a little less.
And third: Some devs probably think that delaying LFR will force part of it's playerbase to try normal/heroic raiding. No idea if it works (it didn't for me).