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It's a good question.
I have a few reasons for using steel on this bike for the forks, and they're understandably personal to me.
Sustainability and permanence
I want to avoid using materials that are lifespan-limited and non-recyclable. That is, on this project. I have other bikes with carbon forks, and they're great. But this is something I'm making which I hope will last for a long while. I have a frame from 50-60 ish years ago and I'm happy to ride it regularly, but in 2073 will someone be happy riding a carbon fork from today? I figure not.
Structural characteristics
Steel is adequately stiff, light and strong. There's an argument for reducing weight by using carbon on a bike which is overbuilt elsewhere, and is ridden in a very physical way, e.g. mountain or gravel. But on a bike primarily for use on the road, I don't feel it's warranted. Also, the bike industry hype around "stiffer is better" is just marketing fluff.
Aesthetics
I think a steel fork looks good combined with a skinny steel frame. It's just my preference, and I also like the look of steel&carbon together but when I thought about the aesthetic for this bike, it just feels right to me for it to be steel-forked.
Novelty
Apparently -- according to the instructor -- we can make this a flat mount disc fitment. This is pretty unusual and in my eyes that makes it kinda cool.
Learning
I'm doing this project to learn how to braze bike stuff. So, making a fork allows me to experience brazing a lugged fork crown. I'd probably not attempt this on my own as the fork is a critical point for failure, and the integrity of a fork relies on good fabrication (e.g. as I understand it, if you mess up a lugged crown it can look fine but in reality have very limited penetration of filler into the joint and thus be unsafe to use). That said, I do not in any way expect to be a framebuilder after one course - but if I can get some tutoring in multiple ways of joining tubes together (fillet brazing, lug brazing) so much the better and more interesting for me.
Just re-read and noticed this. Awesome. Why not carbon?
edit: to add, I dont think it should be carbon, I'm just interested