In "Brave New World," Huxley is introducing an entirely fictional dystopian socity that has many complexities and things to be explained. To his readers, the world that he created would have been very confusing, and he needed a lot of time to set it up, explain how everything was, and set the stage for John the Savage to come on. John the "Savage" is closer to what you and I and Huxley's audience would have related to, as a "normal" human being, and to introduce him right off the bat would not have provided enough contrast with the humans that were being produced in the other society. We needed to fully understand the world that the other people lived in first, before we could bring a more "traditional" person onto the scene. If we didn't understand the setting and all of its implications, then John's arrival from his tribal world would not have been significant to us.
The entire first half of the book is paving the way for us to meet John, and to show a contrast between his ways and the new world's ways. We needed to meet Bernard and Lenina first in order to understand what relationships were like in their world, that way, when John came on the scene with his intense feelings of love, the contrast is more clear. The confusion and unhappiness that Lenina feels at his more typical courtship is understandable, because Huxley spent a lot of time explaining how Lenina and others like her have worked through "relationships" in the past.
We also needed to see Bernard's discontent wtih his society, in order to set up the stage for John to come in and disrupt everything for him, and show him for the shallow, attention-loving person he was at his core. The opening chapters are crucial for a set-up of the actual action of the story; it is like Huxley took the basic plot-map section of the exposition and just drew it out for a much longer time than normal. Most futuristic or science fiction storylines need to spend a lot more time in the set-up, in order to explain the world they are presenting, as it isn't ours. We need to understand the framework of the story before we can understand why any action that occurs in it is significant. I hope that those thoughts helped; good luck!