Why? Because it's such a simple, classic story worthy of any ancient mythology. This is why I love the sixth season so much--it has established its own mythology. My favorite part of high school literature was learning about the stories of gods and familial relationships in Greek, Roman, and Norse Mythology. Lost has created a masterpiece of a back story, telling the tragic story of two conflicting brothers and their intermediate "mother". Yes, it's slow, but that's what I love--getting to know the characters. We can't have 18 "The Candidate" episodes in a season, even at the very end of the show.
What has generated the negative reviews of this episode are attention-hungry cynics. Yes, children actors won't be as easy to direct and this is a TV show with a constrained schedule. Besides the kids, the acting all around has direction and purpose (especially Welliver). I would say the awkward style of talking is appropriate for the setting. We finally see the Man in Black's life before ultimate corruption. When his faith is tested, he gravitates away from his mother toward man, a life of science.
At the end of the hour, I'm definitely not left overwhelmed. Several hours later, though, I think more and more about the hauntingly tragic story and the answers, which are left up to us to correlate. One of my favorite aspects of Lost is that the writers expect a lot of their audience, who is left to ponder Desmond's role, where this light tunnel really is, the rules, why child-Jacob keeps appearing in present time, what Jack's initiation will be as the candidate, etc. Overall, an open-ended, classically-told back story. I am very pleased and incredibly excited for the final 3.5 hours of this masterpiece of a TV show.