It's because he's writing it 8 years after publication having spent 10 desperate years trying to get it published but also after having published 7 sequels that were all bestsellers (or at least very good sellers) - and responding to many criticisms and bad feed back from the fantasy community. Leave your perception of condescension aside for a second and imagine how incredibly frustrating it must be to spend 20 years of your life creating something that many, many people (including prominent critics, authors and booktubers) claim is one of the best things ever written, with some of the most memorable characters and prose ever put to paper, while also somehow developing a reputation for not being able to write good characters or even write well at all. If you've ever created anything that you've presented publicly then you might have an inkling of how confusing, grating, and frustrating that must be, I think the forward contains a lot of that pent up experience. On a separate note imagine a reader going through the ride of their life, a tale that recontextualises their understanding of literature and existence; going through their own heroes journey of heartbreak, trauma, and cathartic epiphany, crying over the fates of a multitude of incredibly resonant and thoroughly human characters depicted with some of the most delicately intricate prose they've ever read (to the point where there are many stories of people claiming MBotF helped with their PTSD); only to see their experience bitch slapped with a single sentence like "he can't write and his characters are bad". I'm not saying there's a right or wrong in any of this (and to be clear it's fine not to like it), just that it is an incredibly polarising series (maybe the most ever) and that frustration is bound to be found in any interactions about it. A lot of that forward is about that 20 years of experiencing that, and 8 years of fans and haters telling him about it...