To make antisocial troglodytes try something new, which they never do anyway and instead post threads complaining.
Well if I was exclusively running LFR i'm not sure i'd want it all to come out at once... There's often a boss that pugs just straight up suck at and I wouldn't want to be stressing trying to get through the whole thing before the reset ended. Also having it come out a bit at a time gives people time to get some basic understanding of the bosses as they come out. By the time the last boss comes out most people should know what to do on the earlier ones so everyone should get through it quicker....
Of course this wouldn't be a problem if people took 5 seconds to read the journal before a pull but they don't and even today you get people asking in chat for the tactics.... I mean seriously... you want somone to make the effort of typing out what to do ? Just look in the journal and it gives an basic overview for each class role...
And I know people are going to say LFR is easy and it is.... but this never stopped people wiping to Siege of Orgrimmar bosses like Nazgrim (before he got nerfed) or Garrosh or Even TOS Kil'jaeden. Just this week I joined a group that wiped twice on Felhounds without even getting close to killing it. I don't know how many tries it took that group to kill it because I left to help my guild raid after the 2 wipes. Would you really want the stress of trying to get through multiple wings in groups like this all at once when nobody knows what's going on (other than the overgeared people who have done it with their guilds)?
It gives people that just run LFR something new for 5 weeks into the new patch. If they release it day 1, then 3 hours later they're 'done' and they'll spent the rest of the 11 weeks complaining that thre's nothing to do. They probably count on N guilds or HC guilds or M guilds to average that time as well
Placating to that group of players is probably one of the mistake Blizzard continues to make in my opinion. Look at all the changes made ot LFR to keep that group happy even though the have no interest in LFR. Except for its removal.
What makes raiders so special that they deserve this kind of attention and treatment from Blizzard? Are they payig higher sub fees?
Can you imagine any other business strategy that involves reducing the quality of a product for one customer base to applease another? Evenmore astounding is the customer base that suffers is the majority?