The reason for that is that Femto's birth is arguably one of the single most important things to happen in that world. I'm not sure if you read the extra chapters with the eclipse but it pretty much explains that thousands of years of history and events have been deliberately planned in order to bring the world to the point it's at now and in order for Griffith to be born. This shouldn't be a huge surprise if you look back at his life you realize the type of person he is and the position he was in has always been larger then life almost, Griffith appears as more of a concept then an actual person, it's almost like being around Griffith is like looking at an epic scaled painting. As the years pass your going to get people discussing Griffiths past as if it's a legend and as if he is some deity descended from heaven, people practically worship the guy, with the exception of Gatts who becomes one of the only people left who truly understands what Griffith actually is. Griffiths entire life was crafted, all of it, to lead him to this point which was why he was so easily taken in by the God Hand's, they already knew exactly who Griffith was, and how to play upon his experiences and desires.
Now imagine the force capable of doing this, directly manipulating thousands of years of history just to create Griffith and to bring the story to this point and ask yourself what could something capable of doing this even want? What is it trying to achieve? If it is possible for this thing to control things to this extent what does that say about Gatts? Would that mean his life was planned out too and that he is only alive for some reason beyond what he is capable of understanding? Many of these are questions even I don't fully understand the answers too yet, although you do start to get a bit more of a sense for them as the story continues. The best part is I highly doubt most of these are things you will be able to guess right away. The world as it is where you are in the story and where I am is so vastly different that you would almost think they have literally traveled to another world and at no point does that change feel like an ass pull or contrived, infact it's a slow gradual change from this point on and it does so in a way I can pretty much promise you will surprise you in a good way, especially as someone I know who loves dark fantasy the way I do.
For me the buildup to the Eclipse and the effect it has is one of Berserks stronger points. It's one of the most fucked up things I've seen in fiction and yet there's not many moments I can point to in any story that effect it as much as this moment effected this series and built on it. No matter where you go in the series now you like Gatts will never truly be able to forget what happened, this is not something that the main character just "gets over" and everything he experiences from this point on whether good or bad will be contrasted to that one moment in his life, which I'll tell you now adds a lot of depth to his character in ways you can't really guess or expect.
**** Sim just binge it all! You have no idea how badly I'm trying to hold back just throwing novel after novel at you regarding all of this X3 I could spend the rest of the evening writing a post on the duality that exists between Gatts and Griffith in the story easily but I really want to be able to discuss the actual plot points specifically with you so you understand exactly what I mean, just know I have a strong itch to talk about this series now and I probably won't hesitate to throw a book -post at you here and there if I get a particular itch for it.
If you feel I'm hinting at too much or giving things away that I shouldn't be just let me know and I'll cut it out, I'm trying to be as vague as possible but it's tough for me to resist talking about it altogether too.